Renewable sources of energy, such as wind, water, and solar power will never run out. These energies can continue and provide power for millions of years. Non-renewable resources such as coal, oil, and natural gas will eventually run out because the Earth cannot make more of them. Solar energy is renewable because the sun always shines. It is also free to use and does not cause pollution. In the future, people will use solar energy because it costs less than other forms of energy. The biggest disadvantage of nuclear power is that it produces radioactive waste. The radioactive waste must be stored safely for thousands of years. If a nuclear plant leaks radioactive materials into the environment, it can cause problems that last a long time.