Cows have "breasts" called "udders" which are joined together in a large sac, often pink in colour, found hanging between the back legs. The udder is divided into four parts, or quarters, each with a large "teat" that the calf is able to grasp with its mouth to suckle from. Cows begin milk production a few days before a calf is born, and can continue to produce milk when bred again and when pregnant with their next calf. Heifers, unless they have given birth to their first calf, do not produce milk. Dairy cows tend to have much larger udders than beef cows, and as such, these type of cows will usually produce more milk than what is needed to feed one calf. "Dairy cows" are female cattle that are raised to produce lots of milk for human consumption. "Beef cows", on the other hand, are female cattle that are used to raise a calf from birth that is used for beef later in its life. Both types of cows will keep producing milk as long as it is demanded, either by the calf, by the milking machine, or by the human that is hand-milking them. When milk from them is no longer needed, they will not explode: they simply "dry up," where the milk they produce is absorbed or taken back in by their bodies. Cows are pregnant for around 9 months, or an average of 280 days.