Why hills are cold and deserts are hot?

Because Hills are way up in the air, and deserts are at the bottom, closer to the Earths Core. Just Kidding
Deserts are Hot and Cold, and the're are cold and hot hills. Take Care

The antarctic is a desert. It's pretty cold there :)

And ice-cream is a desert, let's not forget about that, and ice-cream is cold too.
Just kidding. I know ice-cream is a dessert and not desert. Just testing.

You can get hot and cold deserts. Something is a desert if it has under a certain amount of rainfall a year.
Mountains are cold because they have a high altitude. And for every 100m above sea level you go you lose 1 degree of temperature.
Hills can be anything, they're just a bump in the land.