How to Build an Egg Timer

Egg timers are devices that time how long it takes to boil an egg. These devices will usually count down about three minutes. Traditionally, egg timers were hourglasses. It's easy to make an hourglass egg timer with two glass jars.

To start, take the two soda bottle caps and attach the tops of the two caps using the strong glue. Reinforce this with the electrical tape to ensure that they will not come apart.

Drill a hole through the center of both caps using a drill and bit. Use as skinny a bit as possible so that the hole won't be too large. Once you've pierced through to the other side, you can screw the "double cap" piece onto on of the plastic bottles.

Pour about three-fourths of the sand into the other bottle. If the sand is not clean, you can clean it using hydrochloric acid. (Note: kids shouldn't try this part without an adult) Simply rinse the sand three times with water, and remove excess debris. Then, using gloves and ensuring that you're in a well ventilated room, slowly and carefully add the acid. Do not rush this step so as to avoid the CO2 overflowing. Once the bubbling has subsided, rinse again with water and let dry.

Once you have clean sand in one of the bottles, simply screw the empty bottle onto it. Make sure they are both screwed on tightly. Using a push pin, poke a tiny hole into the side of each bottle so that air can circulate. Then, invert your egg timer and watch the sand flow from one bottle into the other.

Take a stopwatch or a watch with a minute hand and time how long it takes for all the sand to filter from one bottle to the other. For a three-minute egg timer, add more sand if it runs out before three minutes; if you've timed it as taking longer than three minutes, unscrew the sand-filled bottle and dump some out. Once you reach the desired time--give or take a few seconds, of course--you can use this egg timer again and again.