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Birds do it, bees do it, even educated fleas do it; let's do it, let's fall in love.

"Let's Do It" (song) (1954). These words were added to the original 1928 song, replacing lines including "Chinks do it, Japs do it" because Porter realized that those lyrics were offensive.
Birds do it, bees do it, even educated fleas do it; let's do it, let's fall in love.