Discover Density: Build an hourglass to show density differences

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Today’s experiment is fun and captivating! We’ll be talking about density differences between oil and water, and building a fun hourglass to do it.

Since oil is less dense than water, the water droplets flow down into the oil bottle and to the bottom as the oil races up to the top of the hourglass. The two bottles are joined together and have straws inside that help the two liquids interact with one another, creating a beautiful hourglass!

Got some time to give it a go? Let’s get started!

How to make the Discover Density DIY Hourglass science experiment

Supplies you will need

For this experiment, you will need the following:

  • 2 Clear soda bottles, the same size
  • Straw
  • Scissors
  • Duct tape
  • Vegetable oil (enough to fill one bottle)
  • Water (enough to fill one bottle)
  • Hot glue or super glue
  • Food coloring (optional)
  • Funnel (optional)
Supplies needed for the Discover Density DIY Hourglass experiment

Before you start

You may need to keep a towel handy for attaching the two bottles together since it could get messy.


Here is how to do this experiment with your child:

Step 1: Prepare the bottle caps

First up, we are going to prepare the bottle caps. In our experiment, we are joining our two soda bottles by the bottle caps with some glue, then cut holes in the joined caps to allow the water and oil to freely travel between the bottles.

Take off the bottle caps and use your glue to add a good amount on one bottle cap. Join the two caps by pressing them together.

Adding hot glue to one of the bottle cap tops
Firmly pressing together the two bottle cap tops to join them

Then, add a strip of duct tape around those two bottle caps to ensure you have a good seal.

Adding a strip of duct tape to cover the circumference of the joined bottle caps

Step 2: Cut a hole in the joined bottle caps

Using your scissors, take the sharper end of the scissors and create 2 holes in the joined bottle caps, next to one another (see picture).

These holes should be the same size or slightly larger than the straw.

Using a pair of scissors to create the first hole in the joined bottle caps, off to the side
Finished product of adding two holes in the joined bottle caps that are the size of the straw pieces

Step 3: Cut and insert straws

We are going to cut our straw into small 2-3″ segments and fit them into the 2 holes that we created in the joined bottle caps.

When you insert the straws, insert one with most sticking out of one bottle cap and the other mostly sticking out of the opposite bottle cap (see second picture below).

If the holes are larger than the straws, you can use some hot glue to create a seal around the straws. Just be careful not to glue the straws shut!

Two pieces of cut straws, showing the length against my hand
Showing the placement of the two straw pieces in the joined bottle caps

Step 4: Fill the bottles

Before you get started, take off any stickers or wrappers on the bottles so it doesn’t interfere with viewing the experiment.

We’re going to fill one of the bottles with vegetable oil and one with water (you can add food coloring if you’d like). Fill each bottle mostly full, leaving less than an inch before fully filling.

Filling the first bottle with water using a funnel, with only about an inch or two of air at the top
Adding food coloring to the bottle with water
Using a funnel to add oil to the second bottle, with only an inch or two of air at the top of the bottle

Step 5: Attach everything

Finally, we’re going to piece together our density experiment!

Make sure the glue is dry and then screw the cap onto one of your bottles.

Important tip: If you’re using hot glue to join your bottle caps, twist the caps on the bottles very carefully. Otherwise, the glue will get twisted off and you might have a leak.

Placing the joined bottle cap on the bottle of oil and tightening

Here’s where you need to be on your A-game: we have to flip the bottle you screwed the cap onto over so we can screw the cap on the other bottle.

You’ll have to flip the first bottle quickly, or else we’re going to have a spill! I put a towel under it just to be safe.

Once you flip the bottle, quickly screw the other bottle into the remaining bottle cap.

Quickly flipping the bottle of oil over to add the joined bottle caps to the bottle with water

Step 6: Observe the magic!

Flip the bottles so the bottle with the oil in it is on the bottom.

The science behind the Discover Density DIY Hourglass experiment

This experiment teaches:

  • Understanding Differences in Material Properties
  • Hands-On Exploration and Curiosity
  • Density

How it works

The Discover Density experiment teaches about density by comparing oil and water.

Since oil is less dense than water, we see the water droplets falling to the bottom of the oil bottle. Similarly, we see the oil rising to the top of the bottle filled with water. We’re seeing density in action!

The straws in the joined bottle caps allow for the two liquids to interact with one another.

Understanding Differences in Material Properties

This experiment shows kids that different substances have unique properties, like density.

We can observe how oil and water, despite both being liquids, behave differently when placed together.

It’s a great way to teach that not all substances are the same, even though they share some similarities (like both being liquids). Their differences allow them to interact differently with one another like we see with the beads of water falling in the bottle of oil.

Hands-On Exploration and Curiosity

This is a good hands-on activity that has an immediate reaction (the liquids interacting).

It also drives curiosity by wondering what other liquids we can test to see density differences, or what kind of density experiments you can run with other fun props!


You can also take this opportunity to just talk about density in general!

All things have density, but that can look different based on the substance (solid, liquid, or gas). It’s a lot easier to compare density in liquids like we do in this experiment.

You could even compare densities of other liquids, like:

  • Syrup
  • Juice
  • Alcohol

It’s fun to experiment, and you could even experiment just using a container and two liquids!

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