The Missing P

Billy had just learned his ABC's and was very nervous about saying them in front of the class. The teacher told him that the best way to conquer his fears would be to just go ahead and do it. So, trembling, he stood in front of the class and began.

"ABCDEFGHIJKLMNO  QRSTUVWXYZ."

"Very good, Billy. But you forgot the P. Where's the P?" the teacher asked.

Billy answered, "It's running down my leg."