I bought my friend an elephant for his birthday.

This joke gets its giggles from a classic bit of wordplay, taking a common polite phrase and twisting it with a touch of absurdity. When someone says "thank you," the usual response, "don't mention it," is a simple way of saying "you're welcome" or "it was no trouble." But here, the "it" is an elephant, a creature so impossibly large and conspicuous that it would be absolutely impossible *not* to mention it. The humor comes from the sudden shift from the idiomatic meaning to the hilariously literal one.

The setup, giving an elephant as a birthday present, is funny in itself because of how impractical and over-the-top such a gift would be. Elephants are, after all, not exactly known for being easy to wrap or discreetly deliver. This sets the stage for the punchline to land perfectly, highlighting the sheer ridiculousness of the situation. It's a clever little verbal trick that relies on our familiarity with everyday expressions and then pulls the rug out from under them with a jumbo-sized surprise.