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What do you get if you cross a stereo with a refrigerator?
Cool Music

In these jokes two things that we can't or don't usually mix together (for example, a stereo and a refrigerator) are mixed. The result is the punch-line (the part of the joke that makes us laugh) in this case, 'cool music'.
These jokes also depend on the result having two meanings, a usual meaning and an unusual (or joke) meaning. 'Cool music' usually means 'music that's fashionable' but cool also means 'of cold temperature'.

What do you get if you cross a sheep with a kangaroo?
A woolly jumper.
(We get wool from sheep and kangaroos jump a lot. 'Jumper' has two meanings - the usual meaning is a 'sweater made of wool'. The unusual or joke meaning here is to describe 'someone or something that jumps'.)

What do you get if you cross a dog with a computer?
A computer with lots of bites.
(Dogs often bite. Computers have bytes. 'Bite' and 'byte' sound similar.)

A: Knock, knock.
B: Who's there?
A: Lettuce
B: Lettuce who?
A: Lettuce in. It's cold out here!

These jokes involve two people - A and B.
A knocks on B's door because A wants to go into B's house.
B asks A to say who s/he is.
A says her/his first or given name.
B asks for the A's second or family name.
A gives more information and this final line is the punch-line.
The humour of these jokes is that the words in the final two lines sound similar, for example, 'lettuce' and 'let us'.

A: Knock, knock.
B: Who's there?
A: Sam and Janet
B: Sam and Janet who?
A: Sam and Janet evening!
('Sam and Janet' and 'Some enchanted' sound similar. Also, 'Some enchanted Evening' is the first line of a well-known song.)

a cracker (n): a small paper tube which is pulled from each end by two people at Christmas time. It makes a sharp bang when it is pulled and inside there is a small prize and usually a paper hat and a joke written on a small piece of paper

to cross (v): to mix or blend something together

to crack someone up (v, informal): to make someone laugh

to look sharp (v, informal, to talk about someone's clothes): to look good, stylish or fashionable

frostbite (n): injury to skin and tissue from being in freezing temperatures