why do some animals curl into a ball when they are cold?

@ SHUBHAM 
that is a kind of enviornmental adoption they are taking , every animal have diffrent way to take the enviornmental adoption , and with that they will loose their heat from the body and make their body warm . 
and the anathor reason is while curling into spherical shape , it will minimise the surface area , and if the surface area will be minimised , then the maximum heat will be supplied to body . 
HOPE IT CLEARS ..