Touch and temperature

We judge hot and cold by touch every day. In winter you test the bath water with your foot before pouring it. A mother puts a drop of a baby's milk on her wrist to check it is not too hot.

Lambok and his elder sister Phiban live in Shillong. One day after school Lambok says he feels feverish. Phiban touches his forehead and thinks he might have a fever. To be sure, she measures his temperature with a thermometer. It is normal! Touch was not enough.

The book shows why. Take three containers: A with warm water, B with tap water and C with ice-cold water. Keep your right hand in A and your left hand in C for 1–2 minutes. Then put both hands in B at the same time.

• The right hand (from the warm water) feels B is cool.
• The left hand (from the ice-cold water) feels the same water is warm!

Same water, same moment, two different answers. So we cannot always rely on our sense of touch to tell whether something is hot or cold.

A reliable measure of how hot or cold a body is, is its temperature. A hotter body has a higher temperature than a colder body. Hot tea might be about 70 °C and ice-cold water about 5 °C. A device that measures temperature is called a thermometer.