Why does adding one spoon of sugar matter in tea but not in a whole pot?

The same spoon of sugar that transforms one cup vanishes into a full pot - your senses measure change as a percentage, not a fixed step.

Weber's law says the smallest difference you can detect grows with the size of what you started from. Your senses judge change relative to the baseline, so the just-noticeable difference is a roughly constant fraction of the starting amount, not a fixed quantity. Lift a 1kg bag and add 100g and you feel it; carry 20kg and that same 100g disappears. This holds for weight, brightness, loudness, and even price across a usable middle range.

A 5,000d price bump on a 20,000d coffee stings, but the same 5,000d added to a 2,000,000d phone bill barely registers - identical amount, opposite feel.

How big a change feels depends on the size of what it is added to, not on the change itself.

It explains why a small fee tacked onto a huge purchase slips past you, why shops round prices, and why a tiny raise on a big salary feels like nothing - the base sets the scale.

Weber = percent, not amount. One candle lights a dark room; the same candle is invisible among a hundred.

Learn the idea and practice English at the same time.