Two muffins were in an oven...
Two muffins were in an oven. One said to the other "It sure is hot in here!". The other replied "AAAHH! A talking muffin!"
The humor in this joke lies in the fact that muffins cannot talk, and that one of these muffins is aware of this even though he is also a talking muffin.
Normally when someone tells a joke, the listeners suspend disbelief, and accept that in the world of the joke anything that happens is possible. Fantastical elements are taken as a matter of course - talking animals, magic genies, etc.
This joke circumvents that tradition, and pokes fun at the idea of talking muffins, and perhaps muffins in general.