A traditionally egg-timer is a small hourglass, manufactured to measure about three minutes (the time to soft-boil a medium hen's egg in water). As the cooking process is hidden inside the shell, and as changes are rapid in the first few minutes of cooking, a timer is critical. Hourglasses were used as they are mechanical devices designed to measure the passage of time. They involve a flow of a material like sand which is regulated by a narrow neck between two glass bulbs – the material flows, using gravity, from the upper to the lower bulb. Inverting the hourglass repeats the process.