The Laws of Thermodynamics describe the behavior of energy. These laws are extremely useful for determining the energy output of science fiction devices.
First Law: Conservation of Energy
The total amount of mass and energy in the universe is constant. Energy can be changed into different forms or even into matter (or the reverse), but the total remains the same.
This means that if you do 100 Joules of work to lift a rock, 100 Joules of energy had to come from your body to do it, and some of that energy remains in the rock as gravitational potential energy.
Second Law: Entropy
If no energy enters or leaves a system during a change in energy state, the potential energy of the system will always be less after the change than it was before the change.
This means that when you do 100 Joules of work by lifting the rock, the amount of gravitational potential energy added to the rock is less than 100 Joules, the excess becoming wasted heat energy that you can't reuse for rock-lifting.
The Laws of Thermodynamics, especially the First Law, are commonly used for determining the power of science fiction weapons and power sources. By estimating the amount of work done when a weapon destroys an object in a show or movie, you can determine the minimum amount of energy that the weapon transferred to the target. Similarly, if you can determine the mass and change in velocity of a space ship, you can determine minimum power output (and consumption) of its propulsion system.
Trollish debaters often try to find excuses to dismiss the laws of thermodynamics when its implications are not compatible with their preferred conclusions. For instance, the laws of thermodynamics dictate that blasting an Earth-like planet apart requires at least 10E+32 Joules of work to overcome it's gravitational binding energy and a million times more to blast the planet in a fraction of a second. Trolls who dislike what the numbers say about the firepower of a weapon capable of destroying a planet try to invent reasons why the weapon could have accomplished the task with less energy.
Abuse of the 2nd Law of Thermodynamics is a favorite tactic of creationists, who like to claim that it prevents the organization of chemicals into complex organic structures.