Al was his sister, betsy, slide down the slide at the park. she held on to the sides of the slide the whole way down. when they got to the bottom, she noticed that her hands were warm. what caused betsy's hands to be warm?

The answer to your question is Friction

Explanation:

As is mentioned in the question, Betsy held on to the sides of the slide the whole way down, that means that her hands were rubbing the slide and this process causes a force called friction. And Friction is perceived as heat.

The definition of friction is: Friction is the resistance to motion of one object moving relative to another.

In your question her hands create a resistance to motion.