In a fission reaction it's nuclear potential energy that is converted to kinetic energy.
The nucleons are bound together in the nucleus by the strong nuclear force. The total amount of energy it would take overcome this force and disassemble the nucleus to individual protons and neutrons is called the nuclear binding energy.
The same nucleons exist before and after a fission reaction but the total amount of nuclear binding energy depends what which nucleus (or nuclei) they are in. See the graph below.
The average binding of energy of the nucleons becomes greater when the uranium atoms undergo fission and form lighter nuclei. This is to say it would now take even more energy to disassemble all to individual protons and neutrons. It's a bit like falling further down a hole. It now takes more energy to get out.
The nuclear potential energy is mostly converted into kinetic energy of the daughter nuclei, a small fraction (about 6%) of the energy goes into gamma rays and fast neutrons.
The mass of the products is less than the mass of the original nucleus as calculated by $E=mc^2$ but this is not unique to nuclear reactions, the same would be true for burning coal or letting water flow over a dam. It's just that in most non nuclear reactions the change is mass in immeasurably small.
Imagine your cellphone rings. It's tiny speaker is vibrating in and out, producing sound waves that have this form:
Image Source : Tutorvista.com
Because the speaker is just repeating the same tone, unless you have it set to a song, the pattern of sound waves will be a longitudinal wave that has areas where the sound is strongest (compressions), and then areas where the sound is weakest (rarefactions).
It's pretty much the same effect when you answer the phone, but now you are making a more complicated set of compressions and rarefactions because you are talking with far more random sound effects than the speaker produces.
As the sound waves travel through the air, they heat it up very slightly, and that's where the energy that started in your lungs and pushed air through your vocal cords ends up, heating the air around you.
I certainly have heard a lot about the conservation of energy (of all forms) however when it comes to sound in particular, I just can't figure out how it could possibly be recycled into another energy form. When sound (rings of energy vibrating air molecules) is sent, how could that energy get turned into something else? Sure a tiny bit could hit something and turn into a tiny amount of kinetic energy, but doesn't the vast majority of it just fly off into oblivion?
Depending on how dense the air is, the vibrating air coming out of your mouth as you speak will be carried to other molecules, with the further molecules vibrating less as the energy is diluted as heat.
It does fly off into oblivion, which is lucky when its a voice you particularly don't want to hear, all sounds around you get mixed together from different sources and fade away.
So energy is conserved, but what was a concentrated burst of energy when you spoke ends ups as shaking up a large bulk of air molecules, all by a tiny, tiny amount.
My point is that energy is not lost, but rather spread out further and further until it can do no useful work.
Best Answer
According to the conservation of energy, we cannot create or destroy energy, we can only transform it from one form to the other.
So this justifies that gravity doesn't have an infinite source of energy which never runs out! So it must be getting this energy from somewhere else, right?
Let's take the example of a ball dropped from some height. Gravity of the earth pulls it downward, doing work on the ball and giving it kinetic energy. The question you ask is where did it get this energy from?
Go back a step and think about how this ball ended up at such a height? You lifted it up with your arms, and put it on that height. Your arms did work against gravity, spent some energy to put that ball on that height. Where did that spent energy go? This was given to gravity!
When you do work against gravity, you store energy in the gravitational field as gravitational potential energy, which then gravity uses to do work on that object.
In case of hydro power-plants, the sun is giving energy to the water at sea level, to evaporate and rise(in effect doing work against gravity), which ultimately ends up in dams at a higher height, and then falls converting that initial solar energy to electricity!