Pikachu Bus

Listen up, buttercup. This joke is so easy a Magikarp could understand it. The punchline is "Pokémon," because you gotta "poke 'em on" the bus. See? You gotta poke each Pikachu to get 'em on the bus. It's wordplay, which is probably why you didn't get it because you are as dumb as a bag of hammers.