Two snowmen are standing in a field...
Two snowmen are standing in a field. One of them turned to the other and said: "I don't know about you but I can smell carrots!"
This joke is a bit hard to explain. The humor lies in the fact that a snowman's nose is often made out of a carrot, so the snowman is apparently smelling his own nose.
I am incapable of smelling my own nose, so this strikes me as funny. If you were able to smell your own nose, would it be something worth commenting on? I doubt it because you would always be able to smell it due to the fact that it would be the closest thing to your nose at all times.
The absurdity of a snowman being excited about smelling his own nose is where the humor in this joke comes from.
I suppose that it is also absurd that a snowman can talk, and also smell things, but hopefully, that is obvious.