Liquid timers can be very complicated and accurate or very simple and somewhat accurate.This is simple.You will need a plastic baby bottle and something to support it . Add an amount of water to the bottle. Now take an object and very slightly enlarge the hole in the bottle nipple so that the water drips at a slow rate.With that done, now empty the bottle.Now you want to add a measured amount ( it can be any amount but I suggest one ounce because it is marked on the bottle) of water. Turn the bottle nipple end down, support it and time it until the bottle is empty. As an example, if it takes 15 minutes to empty, you have a 15 min. Liquid timer.
