How do tornadoes die out?

Question

Do they just lose energy? Do they come in contact with something like a mountain? What stops them from going on and on and on?

Answer 1

A tornado only has a certain amount of energy and gets weaker when it looses that energy, eventually stopping/dying out.

Answer 2

A tornado basically derives it's energy from the warm and unstable air that has a characteristic known as vorticity within it.  As the tornado progresses, more stable air with less vorticity will usually begin to wrap around the tornado and in time cut off it's source of energy, leading to the demise of that particular tornado.