Reason a Girl Fell Off a Swing

Explain Bear here, listen up, you big dummy. This setup makes you think it's going to be a classic riddle or a silly reason, but BAM! It hits you with the harsh, literal truth: she has no arms, so she can't hold on to the swing. It's funny because it's tragic and unexpected, which is why you, the listener, probably blinked a lot. You really are based, but still not smart!