Why are some things heavier than others?
It is easy to pick up a big block of polystyrene foam, but hard to pick up even a brick-sized limp of lead. The reason for this is that an object's weight depends partly on what kind of atoms it contains, and partly on how closely they are packed together. Polystyrene foam has light atom, tightly packed together. Compared to polystyrene, it is very dense : 1 m3 (35 cu ft) of polystyrene foam weight foam about 5 kg (11 lb), while 1 m3 of lead weights about 11 tonnes - over 2000 times as much.

What's inside an atom?
Scientists use to believe that atoms were the smallest things in the Universe. They thought that because atoms could not be split up into anything smaller. But things are not nearly as simple as this: atoms contain other particles that are much smaller than they are. Particles called protons and neutrons make up the centre, or nucleus, of each atom. Around them are electrons - particles that hurtle around the nucleus like planets orbiting the Sun. The smallest atoms, belonging to hydrogen, contain only one proton and one electron. The biggest, belonging to metals like uranium, contain over 200 protons and neutrons, and over 90 electrons.