I once was sitting outside and watched the birds go by. I checked my watch and said, "My, how time is FLYING by!"
Time Flies
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Explanation
Explain Bear
Okay, listen up, buttercup. The joke is that "flying" has two meanings. Birds fly, and time flies. The guy is watching birds fly by and then realizes time is also flying by, making a pun. Now, I know you probably think birds are just feathery sky raisins, but they're actually evolved dinosaurs. And you, well, you're just... you.