Microplastics are fragments smaller than five millimetres, and most of them are not manufactured at that size. They come from larger items breaking down, from car tyres wearing away on the road and from fibres shed by synthetic clothing in the wash. This matters for policy: banning plastic straws and bags addresses visible litter, which is worth doing, but leaves the largest sources untouched. Reducing microplastic pollution means changing tyres, textiles and the treatment of waste water rather than changing what shoppers carry.