A jack-in-the-box is a toy with a figure (traditionally a clown or jester) hidden inside a decorated box. When you turn a crank on the side, it plays a tune, building tension until suddenly the lid pops open and the figure springs out, often startling whoever's watching.

The toy has been around for hundreds of years. It works on a simple but clever mechanism: as you wind the crank, you're tightening a spring inside while a music box plays. At just the right moment, a latch releases and pop! out jumps the figure. Young children often find jack-in-the-boxes both thrilling and a little scary, especially that first surprise jump.

People use the phrase like a jack-in-the-box to describe someone or something that pops up suddenly and unexpectedly. If your little brother keeps jumping out from behind doors to surprise you, you might say he's bouncing around like a jack-in-the-box. A teacher might describe an energetic student who can't sit still as a jack-in-the-box.