Have a friend copy some daily comic strips from the newspaper and whiteout the captions or the dialogue in the talk bubbles. Now try to write your own captions for each comic strip. You can try to match something that the characters would actually be likely to say or you can be as irreverent as you like.
This exercise is harder than you think, but it's a great way to practice the Comedy Rule of Three, because the typical four panel comic strip uses this rule all the time. Just look at the four panels as set-up, set-up, set-up, and punch line. If you really want a challenge, combine panels from different days. You can also try your luck with single-panel cartoons, either adding dialogue or replacing the caption with your own.