Today many steam engines are still at work. During the 20th century the pistons were replaced by turbines which spin like a windmill pushed by jets of steam. These turn faster with more energy efficiency than the original kinds of piston steam engines. They are used in power plants to operate generators which make electricity. Some ships are also powered by steam turbines. The boilers of steam turbines can be heated by many different types of fuel, even a nuclear reactor in some power stations and warships.