I'm telling Tertius (then about four or five years old) some knock-knock jokes, but he's not getting the idea of the pun. Instead he insists on starting the joke himself, giving me a first name, and then when I say, say "John Who?" he just adds the family name of a friend at his nursery school.*
Trying to be encouraging at all times, I explain that it's awfully nice to remember your friends, but just repeating their names lacks a tad on the laughing-my-ass-off front.
"Look," I say, "what's funny is when something surprises you, when it's silly. For example, if I say I just saw a brown cow, you might be interested, but you wouldn't giggle much." (Tertius could giggle for Britain.) "But if I say I saw an orange cow, you'd laugh because it's such a silly idea."
And he duly does. "I get it," he claims. "Okay, let me try again. Knock knock."
"Apple," he answers. Apple? Where did that come from?
"Apple. . . who?" I ask, with trepidation.
"Orange cow!" he explodes gleefully.
*Alice, the dim verger on The Vicar of Dibley, played to perfection by the magnificent Emma Chambers, did the same thing once, during the regular post-credits joke-telling spot with the equally redoubtable Dawn French.