Why did the matches go to the gym? Because they wanted to get lighter. But at the gym, they also learned how to strike a pose!

And that’s why this pun is fun: This pun plays on the double meaning of “lighter.” Matches want to get “lighter” in weight, a fitness goal, but matches also light fires. The extra phrase “strike a pose” adds humor by relating to the striking action of matches. It’s funny because it combines fitness and wordplay.