Question

Why do we have to shake the thermometer after use?

Answer 1

Answer:

So that the mercury settles on it's original position at the thermometer.

Answer 2

Answer:

Shaking the thermometer hard is what gets the mercury down and ultimately drives it through the constriction so that it rejoins into a single column. In effect, you are making the glass accelerate so fast that it leaves the mercury behind.

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