Because a neutron is needed to instigate a fission (splitting) and neutrons are produced as the product of a fission, these products can go on to produce more fissions. The neutrons produced from these can then go on to produce more fissions etc.etc. Each stage is therefore linked to the previous one and therefore...

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is held in place by the pull of Earth's gravity. The Earth's atmosphere contains a mixture of gases is commonly known as air: 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide - although this is increasing water vapour (average...