A scientist was trying to make a hair grower. But, it didn't work. he added secret ingredients and it worked on a frog, and it grew, so he removed it with hair remover, he threw the hair grower in the toilet, but forgot to flush. and it lived.
It is seen in The Extra Big 'n' Extra Crunchy Captain Underpants Book O' Fun 1 and The All New Captain Underpants Extra-Crunchy Book O' Fun 2.