This is baby's tummy button,.

这是婴儿的肚子按钮。

Other popular answers include "babies come out of your tummy button", "I found them" and "babies are bought in Tesco at night on the top shelf by mums and dads only."

其他常见的答案有“婴儿从你肚脐眼里跑出来”“是我找来的”“婴儿是晚上在超市只准给爸爸妈妈买回家的”。