This one requires understanding a Northern American sport called "ice fishing", in which you go out on a heavily frozen lake or river, cut a hole in the ice, and fish for the pike, bass, perch, etc. that are swimming below the frozen surface. In Minnesota, some guys have little shacks they tow out on the ice, and they go out to fish, drink beer and get away from their wives.

Two guys, Joe and Mike, were out ice fishing one day, and weren't catching much. They noticed a boy about 100 yards away was pulling in one fish after another, so Joe went over to the boy's hole and said "Hey, I'll give you $10 if you trade holes with us!" The kid nodded yes, but didn't say anything.

So they swapped holes, and another hour went by. Joe and Mike were still not having any luck, and the kid was still pulling them in.

Mike said to Joe "He must be using special bait - let's see if he'll tell us his secret."

So they go over to the kid and ask him "What's your secret? How come you're catching so many fish? Come on, kid, we'll give you another $10 if you tell us!"

The kid said "Mmyomm mmtm moo mmeemp muh mmrrms mmrm."

Joe said "What?"

The kid spit a big mess of night crawlers into his gloved hand and said "You've got to keep the worms warm!"