a cage contains branch & birds sit in nests ->  birds sit on eggs  birds sit on eggs & walk on something provides coolness to that something -> a bird sitting on an egg provides warmth to that egg a bird sitting on an egg provides warmth to that egg  an egg may contain a young animal & an animal requires warmth for survival -> an hatched requires coolness to survive an egg requires warmth to survive  a animal walk on an hatched provides coolness to that hatched & an egg requires warmth to survive -> a bird sits on an egg to provide it with warmth for survival