If you could convert all of the energy contained in 1 kg of sugar, or 1 kg of water, or 1 kg of any other stuff, you could drive a car for about 100,000 years without stopping! Why? Albert Einstein, in 1905, wrote down the famous equation E=mc2. It says that mass is a very concentrated form of energy.
Energy is like the 'money' of nature; it comes in two different currencies, and with an enormous exchange rate - the square of the speed of light .
1 kg corresponds to 25,000,000,000 kWh of energy; 1 gr would be enough to supply energy to a medium-sized town for a whole day!
But how can energy be transformed into matter, or vice versa?
Big meteorites traverse our solar system with a typical speed of about 30 km/sec. If such a meteorite enters the Earth's atmosphere, its energy of movement is converted into heat, reaching 100,000 Co or more and melting most of its material ('shooting star').
We do not have the technology to make a space ship go at the speed of light (300,000 km/sec), but it is possible - using accelerators at CERN - to make single particles (like a proton, the nucleus of a hydrogen atom) go that fast.
If a particle moving with this speed hits a block of material, its energy is also transformed, producing 'temperatures' of 10,000,000,000,000 Co or more. Under these extreme circumstances, the energy set free in the collision will transform into matter.
But: what kind of matter do I produce in such collisions?
In a coin factory, hot metal is pressed into coins. They only come in specific sizes and values, as 1p, 2p, 5p, 10p, 50p and 1 pound.
Similarly, nature does not allow energy to be converted into just any kind of matter. Nature has provided us with 'moulds', corresponding to a precisely defined amount of energy, as well as having some other particular properties.
These moulds are analogous to particles, the most important ones in our daily lives being the proton, the neutron and the electron. They have very precisely defined properties, such as their mass, their electric charge or the way they interact with other particles.
So can I transform energy into a single proton or a single electron?
Imagine a hot metal sheet in a coin factory ('energy'). When you stamp out a coin from a metal sheet, you are left with a coin and a hole in the sheet.You could call this hole an "anticoin".
This is similar to what happens when energy transforms into matter. Many experiments have shown that you can only produce a pair of particle and its mirror image, called 'antiparticle', at the same time. Nobody has ever observed the production of only particles, or only antiparticles.
That example also shows another feature observed with particles and antiparticles. To create them, it takes energy, and when you bring them back together ('annihilation', because they disappear into a flash of energy), this energy is released. It is like putting the coin back into the hole, leaving the original metal sheet.
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