system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do cookies that start out as cubes become round when they are baked?
gpt
As it bakes, the cookie flattens and expands in all other directions more-or-less equally. Because it's basically melting rather than growing, it expands from the center of mass and not from the edges.

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Note, this only works with cookies that start out relatively thick and have a good amount of fat. If you cut prepackaged cookies in half, or made cubes out of a leaner dough, they'd end up more square.