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It wasn't originally constitutionally required, but presidents who served two terms have traditionally followed George Washington's example and gotten false teeth.
The older one gets, the faster time seems to fly. For child and teenage me, a US presidential term was basically an eternity; now it seems like the 2020 election happened basically yesterday, and I'm not even old enough yet to run for president there (apart from obviously not being a citizen or resident of the US), so the presidents themselves certainly perceive their terms as even shorter than I do.