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.)