Tanu dips a thermometer into hot water and reads it instantly, within a second — the reading is low

Why

Tanu is expecting the thermometer to know the water's temperature the instant it touches it, but a thermometer does not sense temperature from a distance — it has to BECOME the same temperature as whatever it is measuring, and then it shows its own temperature. When a cool thermometer first goes into hot water, heat starts flowing into it, and for a few seconds it is still warming up, so it reads lower than the water actually is. Give it time and the reading climbs and then settles when the thermometer and the water are equally hot. Test it: watch the liquid or the number after dipping it — it keeps rising for several seconds before holding steady; that steady value is the real reading. Her mother is right: a thermometer has to reach the temperature of what it is measuring, which takes a little time — read too soon and it under-reads.