knock-knock jokes

Jill has been trying to teach Jackson knock-knock jokes. So far her efforts have been a pretty dismal failure. They go a lot like this:

Jill: knock-knock
Jack: who's there?
Jill: lettuce
Jack: lettuce who?
Jill: let us in, it's cold out here!

so then Jack tries to make up his own joke and it usually goes something like this:

Jack: knock-knock
Jill: who's there?
Jack: tomato
Jill: tomato who?
Jack: tomato let us in it's cold out here!

. . . .

but recently he made up a new one of his own and it went like this:

Jack: knock-knock
Jill: who's there
Jack: Jack
Jill: Jack who?
Jack: Jack, don't do that!

...