What is matter made of? If you could see the smallest piece of matter, what would it look like? If you cut a piece of material into halves, and cut one of the halves into halves, and then continued cutting halves into halves - assuming you had a very fine blade and a very powerful microscope - could you continue cutting forever?

Until very recently, there was no microscope powerful enough to see the 'building blocks' of matter. But scientists were able to deduce that there were fundamental particles, which they called atoms (for indivisible), by performing experiments such as firing electrons into targets and seeing how the pieces came out.

Atoms are extremely small particles, out of which all matter is made. They are the smallest particles of a chemical element that still have the properties of that element.

A typical atom is about one millionth of a millimetre across - a million of them laid in a line would measure one millimetre across. The lightest atom is that of hydrogen, while one of the heaviest is that of uranium - about 200 times heavier than hydrogen.

Splitting large atoms into smaller ones or 'fusing' small ones to create larger ones, releases energy - this is what happens inside nuclear reactors and atom bombs (fission) and inside hydrogen bombs and the sun (fusion).