ugrejati

1. to warm; to heat
   Ugrej supu, stižem za deset minuta. — Warm up the soup, I'll be there in ten minutes.
   Ugrejao je ruke iznad vatre. — He warmed his hands over the fire.
2. (ugrejati se) to get warm; to warm up
   Uđi unutra da se ugreješ. — Come inside and warm up.
   Soba se brzo ugrejala. — The room warmed up quickly.

For warming up before exercise, zagrejati se is used, not ugrejati se.