What causes my manure heap to heat up?

It's all about bacteria breaking down organic waste into compost, that stuff you can put on your roses to make them grow nicely. Basically breaking down organic compounds generates heat in the process. It's similar to when our bodies metabolise things, these chemical reactions that are going on create heat. Mainly in our liver, which is a huge source of energy production in the body. A manure heap is a thriving community of microbes all producing heat and similar things happen in farmers hay bales which can get so hot they cause fires!