DIY Density Discovery Bottles

The bottle on the left contains marbles, corn syrup, colored water, glitter, and canola oil. The bottle on the right contains colored water and canola oil, and I've just shaken it.

This tutorial was originally posted on Crafting a Green World way back in 2015!

Use any old water-tight bottles you have on hand to create density discovery bottles for your budding scientist. 

Other than playground sand, which is apparently pig-filthy and will make your ocean in a bottle look polluted, there are a lot of simple, easy, around-the-house ingredients that will encourage babies through big kids (and even adults!) to be excited science explorers.

Glycerin falls through the colored water and rests below.

Discovery bottles put interesting ingredients into a clear, sealed bottle for easy exploration to create a liquid density experiment. 

Although many bottles use colored rice or beans that a kid can shake or shift to find toys hidden inside, density discovery bottles rely on the interesting property of a substance's relationship between its weight and its volume. Two substances with very different densities will not mix, or they'll mix and then settle. 

 And yes, you've seen this while cooking, but my dear Watson, have you ever really observed the phenomenon? Put it in a clear bottle, and you'll be able to. 

 Although you'll want to make these discovery bottles for babies, it's otherwise a very kid-friendly project, accessible even to toddlers. It calls for loads of hands-on, messy exploration, and your kids could easily spend hours at it--my two big kids went back and forth to it all day, then kept their most interesting creations to study the next day, as well.

DIY Discovery Bottles

You will need: 

•  containers. Any clear jar or bottle with a tight-fitting lid will do, although I'd avoid glass, since this project is intended for children. That being said, of course, my children, who are 9 and 10, did much of their exploration in large test tubes. The bottles that we kept, however (the test tubes have long been rinsed and put away), are simply clean plastic peanut butter jars with the labels removed. 
• eye droppers, funnels, measuring cups, a kitchen scale, and other useful tools. These will periodically need to be rinsed or washed off as you work. 
• hot glue. If you're going to keep any discovery bottle for further play, you'll really want to glue the lid on. Really. Glue it on BEFORE your kid goes to stand on the carpet with it. 
• interesting substances. There are so many options here, and they're all going to be found inside your house. Look around for anything that's colorful, and anything that is thin and watery or thick and sticky, but also just grab stuff, and let your kids sort out their physical properties. Here are some ideas:
• alcohol. Rubbing alcohol or vodka and plenty of headspace will allow you to freeze your bottles so that you can observe the effect.
• corn syrup. If you can stand to buy it (you don't have to eat it, but your purchase does support big farming), this this one is a must-have. It's very dense but still flows, and it's clear, so you can really see what's going on with it.
• craft sand. This is apparently a clean sand, but I don't know where it comes from or how it's processed, and I don't own any, so I didn't use it. Sand, nevertheless, would be a VERY interesting substance to include.
• dish soap. Leave plenty of headspace, and your bottle will have an interesting effect when shaken.
• glitter. Not only is the glitter pretty, but depending on the substance, it will fall, float, or mix.
• glycerin.
• marbles. These will fall interestingly through the various densities of substances, especially the corn syrup.
• oils. Unfortunately, the least eco-friendly oil--mineral oil--is also the most awesome for this project, since it's clear. Buy a bottle for the sake of Science if you can stand it, because it's worth the ability to really see the plane of interaction between oil and another substance, but otherwise use any other cooking oils.
• vinegar.
• water. Dye your water with liquid watercolors or food coloring so that you can see it better as it flows among the other substances.

The tutorial for this project is so simple that it isn't even a tutorial at all: all you have to do is play and explore! 

Colored water, dropped from a test tube, sits on top of corn syrup and below mineral oil.

Since my kids were doing this for Science, I required them to keep notebooks that recorded what substances they were mixing, their reactions, and their comparative densities based on these reactions. They played and explored forever in this way, and it was refreshing to see that even in a kid-land that includes, in my opinion, too much Minecraft and My Little Pony, they were thrilled by something as simple as dropping drops of colored water into a test tube half-filled with clear mineral oil. Have you ever tried that? It IS pretty great. 

Corn syrup, itself, is also pretty great. You could fill a bottle only with corn syrup and interesting little objects--marbles, glitter, dice, clean shells--and simply watching each object fall slooooooowly through the corn syrup is quite fascinating. An observant kid (and they're all observant) will notice that the object even leaves a little trail through the corn syrup as it falls. 

Or, fill your bottle half with corn-syrup and half with colored water. Add oil. Add glitter. 

Another never-fail option is simply colored water and oil. You can't dye oil, so if you're not using a clear oil, the trick is to dye your water a color that complements the yellowy, greeny oil that you've chosen. Blue, green, orange... there are a lot of colors that work. Regardless, the colored water and oil behave VERY interestingly together, so you don't want to pass this combo up.

Liquid starch is dropped through mineral oil, glitter, and colored water.

Play around with substances and mixtures to your heart's content, but when you've found a combination that you love, wipe down the bottle's rim and lid with a soapy washcloth and dry it well, then run a line of hot glue completely around the inside of the lid and screw it on the bottle. 

After it's screwed on, you can run another line of hot glue around the bottom of the lid if you're feeling paranoid--and with who knows what inside that bottle, feel free to feel paranoid. 

 Set these discovery bottles on a shelf somewhere that's convenient for you or a kid to return to whenever you want to give them a shake or a turn and watch the pretty colors and interesting interactions. 

Although you'll have used shelf-stable substances, you'll also want to check on them every now and then, and if you see any sign of bacteria growth, obviously pour them out, wash the bottle, and pitch it into the recycling.