Why do Cheerios stick together when they are floating in milk?

This week's question comes from 7 year old Garnett in Toronto who asks: "Why do my Cheerios stick together when they are in milk?"

Dr. Janet Elliott, Professor and Canada Research Chair in Thermodynamics at the University of Alberta explains why is actually known as 'the Cheerio effect". The Cheerios stick together as a way of reducing the surface energy that exists in the milk-cereal system. A single Cheerio floating in milk does three things; it deforms the surface of the liquid, it creates more surface and therefore it generates more surface energy. Two Cheerios floating together deform the liquid surface less than two single Cheerios; so the two together create a lower energy state than two separate Cheerios. By sticking together the Cheerios minimize the surface energy.