It's 2:59pm. The school bell rings in one minute. The teacher is going off about something like "viscosity" or "density" or some other made-up word, but has still forgotten to assign homework. There's a chance you might get off free to build tree forts all weekend long with Jamico. You look at the wall for the clock. It's missing. It has been replaced by a weird double-bottle piece of trash thing. Time has slowed down to a drip. 3pm is approaching one drop at a time. You're so close. Freedom. Science.

What: Liquid Hourglass!
Why: Time drips when you're having fun!
Concepts: density, viscosity, displacement, fluid dynamics
Time: ~ 15 minutes to make or ~ 225 drips, depending
Cost: ~$1.50, mostly for the oil

This project is one that I worked on with Arvind Gupta's team at Toys From Trash. Check out they're amazing work at the link.

Time to drop it like it's hot!

Step 1: Gather Materials

Time to get your act together. Here we go!

Materials:
- Two Bottles (same size with caps) from the recycling
- Two Straws (or one if long)
- Glue (super or epoxy)
- Duct Tape
- Water
- Oil (I used vegetable)
Tools:
- Scissors
- Hot Glue Gun / Hot Glue
- Funnel (optional)

Not too shabby! Let's drip!

Step 2: Prep Your Bottles

A nice easy warm-up! Just cut off those labels, and clean them if they came out of the recycling.

Step 3: Join the Caps

Use some strong glue (epoxy or super) to glue the heads of the caps together (twins!) Hold in position until solid, and then duct tape around the rim to avoid any extra leaks.

NOTE: This is actually a great trick for a lot of science experiments with bottles including making vortices.

Step 4: Insert Straws

Use a pair of scissors to make two straw-diameter holes in your bottle caps that go all the way through. You can also use a drill if you want.

Cut two lengths of straw (about 2") to be inserted into the holes. Add a dab of hot glue to them, press them through, and glue them some more.

At the end, you want one sticking up, and one sticking down with glue all around. Check out the last picture to see!

Step 5: Fill 'Em Up!

It's time for the liquids, and the potential for major oil spills!

Fill up one bottle with liquid to the top, and then give it some food coloring for effect.

Fill the other bottle with oil using a funnel to the top as well. Check out the bubbles within. They're mesmerizing.

Step 6: Attach the Bottles

Here's the great sleight of hand moment. First screw the cap on tight to the bottle filled with oil. Then in one motion flip the bottle filled with oil on to the bottle filled with water, and screw the bottle on upside-down. You will find that if you do it fairly quickly, you won't actually spill any oil. Huzzah!

For your bottles to begin their motion, give the whole thing a flip!

Step 7: Hourglass Drops!

You've done it! And you have yourself a ticking timepiece! Watch as drops escape up from the upper water container down in exchange for oil drops coming up. It is beautiful. They have equal volumes and drop and rise at the same pace as the two bottles will eventually switch all their liquids.

Interestingly in this case, time won't be completely linear as the pressure is going to change as time ticks on. It is still beautiful watching the drops collect on the top and bottom, held by surface tensions until they all join together.

This is a great primer in density and viscosity, and a whole lot more involving states of matter, drops vs. bubbles, and pressure. You can have students time the hourglass on hot vs. cold days, or measure the raising level of water vs. time. There's so much to do. And when you're all done with testing, well, it's just plain pretty. :)

Thank you again to Arvind for your work on this wonderful project.

Show me what you find below! Have fun, get curious, and keep exploring.