Hot colors

[Butterfly32]

what causes something to become hot?

[Rev. Green]

Heat is caused by vibration, so something can become hot if it touches something else which is hot.
Radiation can heat something, so a microwave oven warms food by generating radiation at the correct frequency to cause molecules of water in the food to vibrate.
Heat is produced by work, so stretching an elastic band makes it warmer. (Touch a stretched elastic band to your forehead to check. When you release it it goes cold.)