What: a (well-sealed) bottle that shakes up and separates out
How: You need a leftover plastic soft drink or sports drink type bottle, around the 600ml size-ish makes for good handling, with a lid that can be glued shut. (This isn’t, strictly speaking, necessary… oh, who am I kidding. Glue or tape that f@!#!@ker down good and tight once you’re done.) Fill about a quarter of the bottle with oil, add a capful of food colouring, a guinea pig’s fart’s worth of loose glitter (I don’t know how much that is exactly, but I’m told quite sincerely it’s correct), and then top up with water leaving a small airspace at the top.
This was a baby toy I made for Kid 1 after seeing something similar at an open playgroup-in-the-park in Darwin. The idea is that they can shake it up, roll it around, try and crawl towards it or grab it, fiddle with the textures on the bottle (if your bottle has textures), all depending on age. And, of course, they can watch what happens as the oil and water mix and then separate out again. I found that the glitter tends to stay in the oil, and the colouring stays in the water. I suspect it might be possible to get oil-based colourings that would colour the oil as well so you’d have two quite different colours. I just used craft glitter, but if you were concerned about it being swallowed then edible glitter’s available at cake stores and should work fine for this too. I’ve never bothered trying to explain the oil and water thing to the kids, to me this is just a foundational activity, the sort of thing the kids add to their memory banks of “how the world works” that later on they can pull out and say “Oh, is *that* what that was about”. Such as when a grandparent says knowingly “Like oil and water, dearie” and the kids are all “Like what now?”.
Extras: Really, there’s not a lot more to this, it’s pretty much what it says on the box (but maybe with less syllables than I like to use). There is one modification I’ve seen that could be useful though – using the bottle as a timer. Go to your room, and you can come back out when the bottle’s cleared again. Gives them something to watch, and a known amount of time to spend calming down or getting themselves together or just getting over it (whatever “it” is). My bottle only takes a minute or so to clear, but I think there are recipes online using glitter glue that separate out a little more slowly so you can tweak the timing.