Knock knock joke

Yesterday Jacob had his hand on Rachel's belly, feeling the baby moving. Rachel told him that Lilly was trying to tell him a knock knock joke (you should have seen the "You're being weird" look Jacob gave her):

Rachel: "Knock knock"
Jacob: "Who's there" (giggle giggle)
Rachel: "Baby"
Jacob: "Baby who" (more giggling, you're being weird, mom)
Rachel: "Baby gonna kick you, bam! bam! bam!"

This whole time Sam was off to the side watching, and when the joke finished he got this "look" in his eyes. He came over to Jacob and said, "Hey Jacob, I've got a joke to tell you."

Jacob thought this was great (apparently it's only weird when parents tell dumb jokes). He didn't see it coming at all...

Sam: "Knock knock"
Jacob: "Who's there?" (no clue what is coming)
Sam: "Baby" (still has that "look")
Jacob: "Baby who?" (still clueless)
Sam: (In triumph) "Baby gonna kick you! Bam! Bam! Bam!" At which point Sam really does start kicking Jacob in the shins.

Jacob took it in good humor thankfully.