An egg-timer is a device the primary function of which is to assist in timing the cooking of eggs. Early designs simply counted down for a specific period of time. Some modern designs can time more accurately by depending on water temperature rather than an absolute time. Countdown timers not specifically for eggs are also available for general kitchen and timing use.
Most traditional egg timers have a set time of about 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, so timing is important.