An egg timer is a device whose primary function is to measure a set amount of time. Its primary purpose was to assist in cooking an egg in water. Some later designs allowed adjustments to measure many other cookery tasks. The set time is usually three minutes, that being the approximate time it takes to cook an average-sized hen's egg in water. Hard-boiled eggs take longer to cook. The three-minute egg timer is for soft-boiled eggs. The egg changes rapidly during the first few minutes of cooking. The changes cannot be seen through the eggshell. Therefore timing is important.