Silly / Absurd

#98 · Measure the depth of a mysterious jar

Dip it in. Mark the waterline with your thumb. Measure against a ruler. Mystery mathematically solved.

Sometimes you find a jar and do not know how deep it is. This is rarely urgent, but it is easily solved, and there is a real version of the problem: working out how much is left in an opaque container.

How to do it

1. Slide a chopstick straight down until it touches the bottom.
2. Pinch it at the rim with a fingernail.
3. Withdraw and measure from your fingernail to the tip.

Why it works

This is a dipstick, and it is the same method used to check engine oil and to gauge fuel in a tank. It works because the wetted length of a rod that reaches the bottom equals the depth of the contents, which is one of the very few measurements you can take without any instrument at all.

Tips & warnings

• Mark a chopstick in centimetres once with a permanent marker and keep it in a drawer as a genuine dipstick.
• Two taped end to end reach the bottom of a tall storage jar.