Explanation: Lullabies are the songs that caregivers sing to babies at bed time; the hope of singing a lullaby is that the child will fall asleep. The words “purse” and “bag” are synonyms; a purse is a shoulder bag that women use to carry their items. If the purse falls asleep, the bag would be sleeping. A “sleeping bag” is also the term used for a person-sized bag lined with warm materials that is used for sleeping, especially on camping trips. This joke plays on the phrase “sleeping bag” and the double meaning created in the joke.