Water is made up of lots and lots of little pieces that we call *molecules*. Each molecule in a liquid or gas (like water or like air) is always moving around, all the time. The hotter the liquid (or gas), the faster its molecules are moving.
Because the molecules are moving faster in hot water, they go further into the biscuit, faster. Also, because they’re moving faster, it’s harder for the biscuit’s structure to slow them down, just like a car going fast can crash through a barrier more easily than a car going slowly.
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