Billy Bop was out for a walk one day when he saw some big clouds forming in the sky.   "Looks like rain coming" he said, "I'd better head home!"   But before he could get very far, the rain began to fall, and at the same time a big, beautiful rainbow appeared.   "How wonderful!" said Billy Bop, "I've been told that at the end of a rainbow there's a pot of gold! I think I'll go and see!"