For Ender this year the plan is to make him a set of three “discovery” bottles. I originally thought it would be cool to make him one of those wave machine bottles — you know: oil, water, a bit of food coloring. It’s simple and fascinating enough that it should amuse him for at least a few years, and while he’s learning to crawl, it will double as a rolling toy.

It’s easy enough to make one with an empty soda bottle. I think think this is a pretty typical kindergarten through 3rd grade craft project slash science experiment. I wanted something a little nicer and more permanent than that, and most importantly, bigger, so I could do a couple other things on the inside.

You’d think it would be fairly easy to find a large empty plastic container, but everything shaped the way I wanted either has a lid too large, or is made of glass. Finally I decided a big plastic food container would work since I couldn’t find anything more sturdy, but I couldn’t find any with a mouth large enough. So far I am still without a container, but I think we’ll go to Costco this weekend and I think I’ll be able to find something there.

Inside the bottle, aside from the blue water and clear “air” (oil) I wanted to have a layer of pebbles on the bottom, easy enough, and a boat floating on the water. The boat is where I’m running into trouble.

I KNOW there are things that float on water and sink in oil. I remember it at least vaguely from 10th grade chemistry. What I don’t remember is what those objects are. I must admit that while I generally did well in science, density bottles were not my strong suit. In my density bottle, I added a layer of soap so even my oil and water mixed together.

My first thought was to make the boat out of a walnut shell, because everyone knows walnut shells float, right?

Walnut boats do float on the surface of water, but they float the same way a metal boat floats… based on shape. And like a metal boat, they will not rise to the surface once sunk. This is important obviously, since I can’t guarantee the boat in my wave bottle will stay upright on the surface, and once it’s sealed up, I won’t have anyway to rescue the wreckage.

That pretty much used up my imagination. I expected this to be pretty easy to find out online, but it’s remained surprisingly mysterious. I found a yahoo answers post that gave the relative densities of water and oil, which makes it completely easy because it’s so easy to find out the density of any particular object. Not that I really trust yahoo answers anyway. I also found sites suggesting that “some” plastics would float in water and sink in oil or “some” woods. So nothing simple and obvious.

I found a post somewhere that said wax would work, which seemed plausible. I didn’t think wax would make a very convincing boat though, and I was sort of stuck on the walnut shell, so I came up with the idea of filling the walnut shell with wax.

Then I came up with a brilliant alternative: crayon catamarans.

I bought two packs of crayons (so both sides of the catamaran would be the same color) jewelry wire and a whole bunch of other bits that I thought might work together with the boats, but when I sat down with oil and water and a broken black crayon… it sunk to the bottom of the water glass.

Terribly disappointing. No catamarans. Unless it was just that I used a black crayon… maybe a lighter color wouldn’t weigh as much (yes I’m joking. No need to correct my science please).

I did another google search and someone mentioned a rubber band. Obviously a rubber band won’t make a great boat but it DID give me the idea to try a bouncy ball.

And finally… a possibility.

So I have something that I know floats at the right level, now I just need to play with it some to see how I can make it look like a cute boat.

The current plan is to find a few bright colored bouncy balls, cut them in half, drill a hole through the middle and attach a sail using jewelry wire. I thought I’d use a glass bead to weigh down the bottom and make sure the sail stays pointing upwards, but I may be over-thinking this thing. For the sail I’m using a bit of fabric, and I’ll polyurethane the whole thing to keep it somewhat water safe.

I’ll post an update as soon as I figure it out.

Meanwhile, enjoy my rubber Saturn.

*First image from Flickr user Sean Rogers1. All following from me.