Why your eyes lie about glasses

By Alex · 1 November 2026 · 5 minute read

The science behind Pourdle, and why "fill it to half" is harder than it sounds.

Fill a real cocktail glass to half its height and you'd think it's half full. It isn't. It holds about one eighth of what the glass can take. Your eyes didn't make a small mistake there. They made a huge one, and they make it every time.

Pourdle is built around that mistake. Every day you get five glasses with no markings and a target like "fill to 63%". That number is about volume, not height. Here's why that's so hard to judge by eye.

Your brain measures height, not volume

When we look at liquid in a glass, our brain mostly checks one thing: how high the surface is. Height is easy to see. Volume isn't, because it depends on how wide the glass is at every point from the bottom to the top.

Children show this most clearly. In the psychologist Jean Piaget's famous experiments, a child watches the same amount of water poured from a short, wide glass into a tall, thin one. Most young children then say the tall glass has more, because the water reaches higher. Piaget called the missing idea conservation: understanding that the amount stays the same even when its shape changes.

Adults pass that test, but we never fully lose the habit. Researchers in consumer psychology have found that people consistently judge tall, thin containers as holding more than short, wide ones of the same size. The height of a glass grabs our attention far more than its width. (A well-known paper on this is Raghubir and Krishna's "Vital Dimensions in Volume Perception" from 1999.)

Width matters more than you'd think

Here's the part your eyes get wrong. When a glass gets twice as wide, it doesn't hold twice as much at that height. It holds about four times as much, because the liquid spreads out in two directions: side to side, and front to back.

Pourdle's glasses are drawn flat on your screen, so there the amount works like the area of a flat shape: twice as wide holds twice as much, not four times. The same cone filled to half its height holds a quarter, and it's half full at about 71% of its height. That's gentler than a real glass, but it fools your eyes in exactly the same way.

Either way, the wide parts of a glass swallow far more liquid than the thin parts:

• In a tumbler that's wider at the top, the last few centimetres hold a lot more than the first few. Filling it to 50% means stopping noticeably below the halfway line.
• In a cocktail glass (a cone with the point at the bottom), the bottom half of the height holds only a small part of the drink: 12.5% in a real glass, 25% in Pourdle. To make it truly half full you have to go much higher: about 79% of the height in a real glass, about 71% in Pourdle.
• In a glass that bulges in the middle, like a lava lamp or a vase, the belly drinks up liquid while the level barely seems to move. Then the narrow neck fills almost instantly.

That's why the same target can feel completely different from one Pourdle glass to the next.

Why the pour itself makes it harder

Even if you've judged the shape well, the pour is working against you:

• The flow speeds up. In Pourdle, the longer you hold, the faster the water runs, just like opening a tap wider. A late stop costs more than an early one.
• Water is still in the air. When you let go, the water already falling still lands, and a few drips follow. You always have to stop a little before the line.
• You can't pour water out. You can top up, but you can't undo, so overshooting is the one mistake you can't fix.

How to get better at it

A few habits that help in Pourdle (and at a real bar):

1. Look at the shape before you pour. Find the widest part of the glass. Most of the volume lives there.
2. Think in thirds, not halves. Ask yourself where one third and two thirds of the liquid would sit, then aim between them.
3. Pour most of it in one go, then use short taps. Short taps are slow and safe, so they're perfect for the last few percent.
4. Stop early. Water in the air and the last drips will add a little more.
5. Learn from the results screen. After each round you see your level next to the target line. Notice whether you tend to go over or under on wide tops, narrow necks or bulging middles. That pattern is your personal blind spot.

Try it yourself

Your eyes will keep measuring height, but you can train them to notice width. That's what the daily game is for: five glasses, five targets, no markings. For the special glasses, see how each twist works.