The cuckoo is a bird famous for one of nature's strangest behaviors: it lays its eggs in other birds' nests, tricking them into raising its chicks. A mother cuckoo watches until another bird leaves its nest, then quickly sneaks in, removes one of the original eggs, and leaves her own egg behind. When the cuckoo chick hatches, it often pushes the other eggs out of the nest. The unsuspecting parent bird, working harder than ever, feeds the growing cuckoo chick as if it were its own, even though the imposter grows much larger than its adoptive parents.

Because of this behavior, we call someone cuckoo when they're acting foolish or crazy, or describe something absurd as cuckoo. If your friend suggests wearing winter coats to the beach in July, you might say, “That's a cuckoo idea!”

The cuckoo also gives its name to cuckoo clocks, those charming wooden timepieces where a little mechanical bird pops out on the hour and calls “cuckoo, cuckoo!” The real bird makes a similar two-note call that sounds like its name.