Panel 1: Having hammered together her cruder birdhouse, Liz shows it off to John and tells him that she's done. He says that it's not bad for a first effort but she forgot something.
Panel 2: When she asks what she forgot, he tells her "A hole for the birds to go in and out of."
Panel 3: She knows this but wants to wait a while to decide something. He sets the punchline by asking if she's trying to decide what kind of bird it'll be for (which means that he'll talk over her head when he drills the entry hole).
Panel 4: When she says "Until she decides which side is the front", John laughs out loud.
Summary: This does not appeal to me too much because the last thing you should do to someone who just plain doesn't understand something is laugh at their confusion and ignorance.