In the comments of this post about chemistry sets and science education, gwdonnelly asked:
As a kid I loved playing with tools, fire, magnifying glasses, etc, etc. Along with some mates I made thermite and even had a go at some very small touch powder (could do with more practice at growing crystals there!)...
Anyway, I would like to get my kids into doing experiments in a slightly more controlled, and safe, way - any recommendations on what to get a 4-5 year old started with?
I've made this a new post so that other commenters can chime in with ideas. Here's what I managed to think of:
(source: Flickr user futileboy)
3: While you're at it, the cheap-'n'-lazy version of a chemical garden, those little cardboard trees that grow fuzzy crystals:
(Source: Flickr user drewish)
(Source: Flickr user watz)
4: Go for a wander and collect and identify rocks, plants and other people's unattended property. (Strike out whichever does not apply.) You can build a collection of a wide variety of rocks you can't find in your own neighbourhood quite cheaply via eBay, too. Just bear in mind that if a mineral sample seems too good to be true, it's eminently possible that it is.
5: Tumbling your own rocks has been a popular hobby for ages, too; all sorts of ordinary-looking rocks come up lovely when highly polished:
(Source: Flickr user vpickering)
You can make your own tumbler (or "ball mill", which is only a ball mill if you... put balls in it) from a plastic container and a scrounged-up motor. All you're likely to have to buy, besides perhaps a grab bag or two of guaranteed-impressive un-tumbled minerals, is some "tumbling media", so you can have fast abrading of rough stones and fine polishing later on without just hoping a handful of sand will do both jobs. (There are some other inexpensive tumbling-media options, too.)
6: Five years might be a bit young for soldering or an actual microcontroller (look how cheap!), but you can still play with electronics - wires, motors, batteries (and/or a jimmied PC PSU), switches (you've obviously got to have at least one knife switch)...
(Source: Flickr user LenP17)
(image source Flickr user c3o)
(Source: Flickr user Trocaire)
8: Growing mustard/cress/bean sprouts on a wet paper towel...
(Source: Flickr user kidicarus222)
...or the classic toothpicked spud.
(And even then it's no big deal, unless they swallow more than one. This has recently turned into a problem for people who sell small rare-earth magnets as toys in the USA, because apparently you can't trust an American child under the age of 14 not to eat everything they touch. See also the American Kinder Surprise ban. Apparently something magical happens between the ages of 14 and 18, which transforms American children from Lego-eating lackwits into citizens responsible enough to be trusted with a firearm. But not a beer until they're 21, of course!)
Do make sure you stick with small rare-earth magnets for toys. Obviously really big rare-earth magnets can crush your hand, but much smaller ones can snap together hard enough that they break. Don't get any very thin ones, and don't get anything with a diameter much more than a centimetre (half an inch, say), and their field is small enough and their momentum low enough that they'll last a long time.
If you want safe big magnets, get simple and cheap black ferrite ones instead; they're much weaker than rare-earth magnets. (It's theoretically possible to lever the big ferrite ring magnet off the back of a speaker driver, but only once have I managed to do that with a magnet of any size without cracking it.)
10: Looking at stuff under a microscope. A proper lab microscope would be best but those sell for pretty large prices, and the cheap small ones for kids are, I think, usually pretty crappy quality. Instead, you could go for one that plugs into a TV:
(Source: Flickr user Neven Mrgan)
A cheap alternative is, of course, your basic magnifying glass, or a "loupe", which is either a small high-powered magnifying glass, or a monocle-style mad-scientist magnifier.
(Source: Flickr user rouwkema)
(One of van Leeuwenhoek's greatest, but least helpful, achievements was concealing how easy it is to make his microscopes' tiny lenses. Everybody thought he ground them with fantastic accuracy, when all he actually did was melt the end of a glass rod and allow surface tension to pull it into a sphere.)
Leeuwenhoek microscopes aren't the easiest to look through, but can effortlessly resolve the tiny beasties in pond water.
Oh, and then there's the quickest microscope ever, provided you have a digital camera with a very small lens, like the camera in a phone: Just put a drop of water on the lens, turn the phone over carefully...
(Source: Flickr user ipasha)
...and bingo, one microscope!
OK, folks; what have I missed?