meniscus

The meniscus is the curved surface a liquid forms inside a narrow container, like a measuring cylinder, instead of sitting perfectly flat.

• For water and other colourless liquids, the correct reading is taken at the bottom of this curve — looking at eye level with the liquid surface to avoid a skewed reading.
• Coloured liquids flip that rule: their meniscus is read at the top of the curve instead, since the coloured liquid itself makes the bottom of the curve hard to see clearly.
• The meniscus forms because of how liquid molecules interact with the container's walls compared to each other — a subtlety that becomes important precision, not just trivia, once you're trying to measure exactly 50 mL of something.
• Ignoring the meniscus and just eyeballing the liquid's surface is one of the most common sources of small measurement errors in a school lab — a narrow, tall measuring cylinder makes reading it correctly much easier than a wide, short one would.