What are cool themselves?

"Cool themselves" means something that stays cool by itself, without needing help from outside.

Imagine you have two cups of hot chocolate. One cup feels warm to your hand after a while, but the other one still feels cool, even though it was also hot at first! The one that stayed cool is like something that cools itself.

Think of your favorite drink in a metal can. When you take it out of the fridge, it's cold, and it stays cold for a long time. That’s because metal is really good at keeping things cool, kind of like a superhero that doesn’t need any help to stay chilly.

Cool themselves are useful in real life too! Like when you have a cold drink in a plastic bottle, it might warm up faster than one in a metal can. So, if you want your drink to stay cool longer, you pick the right kind of container, just like picking the best superhero for the job!