What is it with kids and the word...

POOP! All the way home from work last night the kids could not stop saying it! Ellie was saying she knew how to spell it and then they were putting poop in front of every word they could think of...poopy Evan, poopy brush, poopy head...you name it they added it! They have done this a few times now, laughing like crazy at themselves...I just don't get it! Is it that funny? Or is it a word that is just slightly naughty so they can say it without getting soap in their mouth but it still pushes my buttons?!