Make a Reverse Hourglass

Materials you'll need

• 2 glass bottles with matching neck openings
• Multicolored foam pearls
• Water
• Stiff cardstock or thin plastic
• A small pitcher or measuring cup

 

Step-by-step tutorial

Step 1: Fill one bottle all the way to the top with multicolored foam pearls.

Step 2: Fill the second bottle with water, right up to the very rim — no air gap at the top.

Step 3: With a grown-up's help, take both bottles outside or somewhere spills won't matter, since a little water may splash out.

Step 4: Cover the mouth of the water bottle completely with the cardstock or plastic sheet, then carefully flip the bottle upside down while holding the cover in place.

Step 5: Set the flipped water bottle directly on top of the pearl-filled bottle, lining up the two openings so they match up exactly.

Step 6: Slowly slide the cover out from between the two bottles, and watch the foam pearls rise up into the water while the water sinks down below.

Learn more

This project is called a reverse hourglass because it flips the usual sand-timer rule on its head. In a normal hourglass, sand always falls downward through the narrow neck, no matter what. But foam pearls are less dense than water — meaning a scoop of foam weighs a lot less than the same-size scoop of water — so buoyancy, the upward push water gives to anything less dense than itself, wins out over gravity here. When the two bottles connect, the heavier water sinks down through the neck while the lighter foam pearls get pushed up and out of the way. That's why the "sand" in this hourglass rises instead of falls.