Mac pro becoming hot

How hot?

Hot where on the MacBook?

What is the Mac doing when it gets hot?

Running videos, especially Flash ones makes the Mac work harder and get hot

You can go to the Mac app Store and get apps that will display the various temperatures.

Heat is a function of the number and types of applications in use. Install a temperature application as IIIaass suggests and open Activity Monitor to see what applications are using the most resources.

Note that the aluminum case is part of the heat dissipation system of a MBP and it will get hot when required and that is normal. What you may perceive as a problem may not be one. Without specific data, no valid judgement can be made.

Ciao.

Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity - Apple Support