The letter "F" is difficult for my students to pronounce. More often the not, it comes out as "P". I gave them a script for a role-play exercise that featured a troll. In order to give each student a speaking part, I gave the troll the line:
"Fee Fi Fo Fum!"
As it turned out, my kids had never heard the expression. With complete seriousness, they read the line as:
"Pee Pee Po Poom!"