A jack-in-the-box is a children's toy that outwardly consists of a box with a crank. 

When the crank is turned, a music box mechanism in the toy plays a melody.

After the crank has been turned a sufficient number of times (such as at the end of the melody), the lid pops open and a figure, usually a clown or jester, pops out of the box.

Some jacks-in-the-box open at random times when cranked, making the startle even more effective. Many of those that use "Pop Goes the Weasel" open at the point in the melody when the word "pop" would be sung.