Simplify the following text for a Grade 7 native English-speaking person.
Original: Weasels vary in length from 12 t , and usually have a red or brown upper coat and a white belly ; some populations of some species moult to a wholly white coat in winter .
Simplified: Weasels length is from 12 to 45 centimetres ( 5 to 18 in ) , and usually have a red or brown upper coat and a white belly .