With some cups, you can lift them and take a small sip naturally. With others, you need to raise your wrist a little further before the coffee reaches your lips. Before you know it, that sip is larger than you intended.
If the coffee comes from the same pot and the brewing recipe has not changed, where does that difference come from? Beyond the drink itself, there is a brief part of drinking that we rarely notice: the coffee travels from inside the cup to the rim, then into the mouth.
That sip was what we wanted to work on when developing Coffee Stage Sharing Cup. We wanted the rim to feel less bulky against the lips, and the coffee to flow into the mouth at a more continuous, comfortable pace when taking small sips. So we kept exploring a body that curves inward, then flares outward as it approaches the opening.
But simply saying “a flowing cup shape makes coffee feel smoother” leaves out the most important explanation. What does the curve do? Where does surface tension come in? To answer those questions, let us slow down the coffee’s passage to the lips.
The amount in the cup affects your movement before the coffee reaches your mouth
Imagine the same cup, once fairly full and once with only a little coffee left at the bottom. Even with exactly the same cup, you would struggle to drink from it using the same lifting motion both times.
When there is more coffee, its surface is closer to the rim. When less remains, you often need to tilt the cup further before the coffee reaches the drinking position. So before discussing cup shape, it helps to ask: how much was left in the cup for that sip? If one cup is full and the other contains only a little, the difference you feel already includes the amount poured.
Left to right: S Size, M Size, L Size, Alpha Size, Beta Size.
The outlines of Sharing Cup S, M, L, Alpha and Beta each serve a purpose. This diagram first helps us identify where the body curves inward, where the rim flares outward, and the central aroma column inside Beta. It does not rank the five by how quickly they pour.
As you slowly tilt a cup, the position of the liquid surface relative to the rim changes. Close to equilibrium, the surface stays roughly horizontal; when the cup moves quickly, the liquid sloshes. What looks like a still, horizontal line also involves the movement of your wrist when you actually drink.
Top row, left to right: S Size, M Size, L Size. Bottom row: Alpha Size, Beta Size. The lower-row diagrams use the recessed side for drinking.
This is why a side view can help explain the shape, but the angle in a diagram alone cannot tell us how much coffee each person drinks. The amount in the cup, the speed of lifting it and the position of the lips are all still changing.
Liquid needs to leave the rim as well as follow the wall
As coffee approaches the opening, it reaches the region where the wall curves inward and then flares outward. We included this contour in the design with the aim of supporting the way coffee enters the mouth in small sips. The diagram below separates the stages to make the relative positions of the liquid and cup wall easier to see.
From the top left: ① at rest; ② liquid surface near the waist; ③ front edge reaching the flared rim; ④ entry position.
Watching liquid move along the cup wall, it is easy to wonder: does closer contact always make for a smoother sip?
The end of a pour of tea or water offers a counterexample. The stream slows down, then the last of it curls around the outside of the spout and drips down the outer wall. The liquid is indeed following the surface; it simply has not left where expected.
In water-flow experiments on the “teapot effect”, Duez and colleagues found that surface wettability, edge curvature and flow speed jointly affect how the stream separates. Under their experimental conditions, rounder, thicker edges were more prone to dribbling. The study did not test coffee drinking, but it reminded us to consider guidance inside the cup and separation at the rim as distinct questions. Read the original research
So the design cannot stop at whether the cup’s waist looks attractive. That final short section of rim near the lips also deserves repeated trials. Where the cup takes the liquid and how a person drinks it need to work together.
What does the thin region at the front of the diagram mean?
In a side view, a horizontal liquid surface meets a tilted cup wall to form a gradually narrowing region. This helps explain where the liquid first approaches the opening, which is the design idea we originally wanted to share.
However, the distance between those two lines cannot be taken directly as the thickness of the flowing coffee layer. Actual drinking also involves speed, deformation of the liquid surface and contact with the lips. We have no measurements here of the coffee-layer thickness at the Sharing Cup rim. The diagram offers a way to understand the idea, rather than a value in millimetres.
To think about surface tension, start with a drop of clean water: small droplets often form a rounded shape, while larger ones are more readily flattened by gravity. MIT’s teaching material on capillarity uses this contrast to explain how the effects of surface tension and gravity change with scale. The capillary length in that material is not a thickness that flowing water must form. View the teaching material
“Surface tension” and “wetting” should not be treated as the same thing either. The former concerns how a liquid surface holds its shape; the latter describes whether liquid tends to spread out or gather together when it meets a solid surface. Understanding flow at the rim also means considering gravity, speed and the local shape. Saying “surface tension makes it smooth” skips everything in between.
A thin rim first changes what touches your lips
When you take a sip of coffee, your lips first touch the rim. How noticeable that contact is, how far you open your mouth and whether you need to adjust your posture are all part of drinking.
Sharing Cup uses a thin rim to reduce the feeling of bulk as you drink. Its flowing body shape is a separate design direction, intended to work with the coffee’s passage towards the opening. The two meet in the same sip, but do different things. A thin rim alone does not mean that coffee must flow more slowly or be more enjoyable to drink.
The circle on the right enlarges the relative positions of the rim, lower lip and tongue. Arrows express only the original design concept.
In everyday tasting with small sips, we find it easier to notice the rim’s contact and the feeling as coffee first enters the mouth. That continuity is what the design has been seeking. The side-view lines express this direction; they do not measure the path of coffee inside the mouth or mean that liquid spreads evenly across the tongue.
A 2026 study of rim hardness had 30 participants taste drinks under different rim conditions. According to the publisher’s abstract, some comfort and flavour ratings changed, but the effects were not consistent across all conditions. It tested gummy-candy and silicone rims, so it cannot prove that thin ceramic cups are better. It does offer a useful reminder: a change in perception can begin with contact at the lips. Read the research abstract
If you divide coffee between two cups and one feels “smoother”, try being more specific. Is the rim more comfortable against your lips? Is the sip easier to control? Or does the coffee feel heavier and more coating once it is in your mouth? Describing it clearly helps you understand what you like, without rushing to attribute it to one particular curve.
Before your next coffee, try the cups you already have
You do not need to buy a new cup first. Take two cups you use regularly. Using room-temperature water over the sink, observe how far you need to tilt the same cup before water starts to pour when it contains more or less water. Move slowly, then try again at your normal pace. Just watch the liquid; there is no need to drink while pouring.
For coffee, wait until it is at a comfortable drinking temperature. Divide coffee from the same pot into similar amounts, alternate small sips between the cups, and switch which cup you try first. Three things are enough to notice:
- Is the rim comfortable when it touches your lips?
- Without deliberately slurping or raising your wrist further, is the sip the size you want?
- Once the coffee enters your mouth, does it feel light, heavy or have some other texture?
Temperature, expectation and movement all affect this everyday comparison. It can help you find your preferences, but cannot establish a particular physical cause on its own. If your existing cup already makes coffee enjoyable, keep using it. If you begin to notice contact or a drinking rhythm you do not like, you will have a clearer direction than “something smoother” when you next choose a cup.
That same curiosity about understanding a feeling led us to start changing angles and proportions on a computer. The next challenge was harder than drawing the curve: how do you turn a digital cup into something you can actually drink from?
