Monotremes are thought to have evolved from reptiles and were the ancestors of modern mammals. The only living monotremes today are platypuses and echidnas. They are warm-blooded, have hair, and produce milk, so they are mammals. However, not like other mammals, they lay eggs. Unlike most birds and reptiles, while the egg is still inside a mother monotreme, she supplies it with a small amount of nutrition from her own body, similar to other mammals. Monotreme eggs are small, white, and spherical. They are laid in small numbers and are fastidiously cared for by their mother in her burrow until 4 to 6 months after hatching. Platypuses keep their eggs warm by curling their tail over them whereas echidnas warm them by tucking them in a small fold of skin across their stomachs. Monotremes do not have nipples to produce milk from, so instead they sweat milk which their newly-hatched young drink.