Strangely enough, the children didn't believe me, that the ancient Egyptians dealt with the problem of telling time on cloudy days, by taking naps (see our sundail post, here). So, we moved on today, to the next innovation in ancient time telling: the clepsydra (Greek for water thief), a water clock used by the Egyptians as early as 2000BC.
To make one yourself, you need a paper, or plastic cup, a tallish jar, with a small enough mouth to hold the cup, but not let it slip through, a pin or needle, a marker, and a stopwatch, or clock with second hands (if you want to get really authentic, you can make yours on a sunny day, and use a sundial to time the water drips - we were completely clouded in today, so we used a clock).
Use the pin to make a small hole in the bottom of the cup.
Then, test the hole, to make sure water can drip through it at a nice rate. We found an embroidery needle made a pretty good sized hole.
Then, mark the spot you're going to fill the cup up to, place the cup in the mouth of the jar, and fill it up.
Watch the time closely, and mark the water level of the jar at given intervals. Remember, the water will leave the cup faster at first, due to the greater amount of water pressure when the cup is full, opposed to when it is empty, so your lines will get closer together as more time passes.
It's also a good idea to have someone keep an eye on the level of the water in the cup, to make sure it doesn't run out while you're still timing.
To find out more about clepsydras, check out this Encyclopedia Britannica article on the subject, or watch the Ancient Hobbyist build a slightly more complicated version.
It's great to be a homeschooler.