Fact 1: Friction causes the temperature of an object to increase.

Fact 2: Friction in bearings causes an increase of the temperature inside the bearing.

Given the two facts above, answer the question "what can cause bearings to heat up?" with the following options: 
- friction
 - loose soil
 - energy
 - voltage
 - Energy.
 - wind
 - electrically
 - sweating


 

 friction        