Using magnetic ink and any oil lying around your house, make a substance that's liquid when it's sitting around, but turn solid in the presence of a magnetic field
Step 1: Intro
This instructable will show you how to make your own ferrofluid. A ferrofluid is a fluid with magnetic particles in it, and if the fluid is exposed to a magnetic field, all the magnetic particles will align with the field lines, and making the fluid much more dense. There's a lot of cool things you can do with this fluid. Enjoy!!!
Step 2: Get the Materials
This is just about the simplest ferrofluid you can make. You'll need two basic materials: magnetic (MICR) ink, and a household oil. I've tried a couple types of oils, and it seemd like a light lubricating oil works best, but any cooking oil will work fine, as well. The amount of oil you have is pretty much the amount of ferrofluid you'll get out--about 50mL is good for starters, but feel free to make as much as you want.
The ink is a dry magnetic ink that's used to print checks and other documents that use magnetic character recognition (check out the wikipedia article at http://en.wikipedia.org/wiki/Magnetic_ink_character_recognition if you want to know more about this awesome technology). You can get a box of it on ebay for about thirty bucks (search for MICR or magnetic ink), and you can also get it at http://www.expresstonercartridges.com/okidata.htm. It's going out of style, so you'll have to poke around a little bit to find it. It's important to remember that you don't want a toner cartridge--just the toner.
Step 3: Mix It!
Pour some oil into a mixing cup. Add a bit of the ink, and stir it into the oil. You're making a suspension, so the ink won't dissolve in the oil. Just stir it. Keep on adding ink and stirring until you have a thick solution. There's no exact science to this. A good guideline for identifying a well-mixed fluid is that if you tip your mixing cup, the fluid should ooze rather than slosh.
Step 4: Play With It!
Grab your favorite permanent magnet and hold it up to the mixing cup (don't touch ithe fluid with the magnet, or even get close, unless you enjoy cleaning). Watch how the fluid turns from liquid to solid as you bring the magnet closer. Show your friends the horrible oily liquid you have in a cup, and then simulatenously slap a magnet onto it as you harmlessly 'dump' the cup over their heads (practice this first). Buy some cheap solenoids from http://www.allelectronics.com and electrically change the consistency of the fluid. Just enjoy!
Step 5: What Other People Do With It
There's a lot of cool things people do with ferrofluid.
You can make brakes with it by putting some fluid between a wheel's axle and hub. The wheel will spin freely as the fluid acts just as a liquid lubricant, but if you apply a magnetic field, you're suddenly putting a lot of friction of the wheel's rotation.
The good folks at the Univeristat Der Kunste Berlin made a ferrofluid display that can play Nibbles: http://www.digital.udk-berlin.de/en/projects/winter0405/main/hauptprojekt/snoil.html
I'm building on-the-fly braille translators and tactile interfaces by sensing if people are moving the fluid: http://www.artiswrong.com/ffb
(just pictures and movies, no explanation/writeup yet....)