Asked by: David Kelly, Manchester
All protons are positively charged, and as a result repel each other. So the fact that atoms even exist points to a force able to overcome this repulsion.
Called the strong interaction, its origins lie in the particles lurking inside both protons and neutrons, called quarks. These possess a weird form of charge, whimsically termed ‘colour’, which glues them together inside their host particles – and also seeps out to bind the protons and neutrons together, too.