Science Buddies: "Ask an Expert"

I may not be a physics expert, but I know the basics. Temperature is defined by the velocity of the atoms. If atoms are moving in a high speed there is a high temperature. If the atoms are moving at a low speed, there means they are in an environment with a low temperature. I get that, however, what is the cause of the movements of these atoms. What makes them go at a certain temperature? I mean I definitely know that atoms don't move randomly saying they want to go fast or slow, there is something that makes them go at a certain velocity. But what is it? Many scientists have made multiple experiments trying to define temperature. Some people thought it was a different molecule that created that, but the final conclusion is the speed of the atoms. But what causes this change of speed. Yes, the velocity in atoms may be the action that happens when atoms reach a certain of temperature, but something has to cause this, right? Just my curiosity getting the best of me.

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