A magnetorheological fluid is a liquid that hardens near a magnet, and becomes liquid again when you remove the magnet. They are simple to make in your kitchen after a trip to a sandbox.
In the introduction to this section I described how to mine iron ore out of the sand from playgrounds or the beach. You may want to spend a while at the beach, because we will need a good handful of ore.
The ore that sticks to the magnets in the plastic bags has quite a bit of sand entrained in it. We can remove the sand by some additional refining. Make sure the ore is dry. Spread the ore out on a paper plate, and hold the bag with the magnet over the plate until a small amount of ore jumps up to the bag. Put this ore onto another plate, and continue until no more ore rises to the magnet in the bag. Don't get the bag too close to the plate, since there are many sand grains with ore stuck to them. We wish to keep only the ore that does not stick to grains of sand. The ore in the second plate should be visibly darker than what is left in the first plate. If you can see a lot of sand in the second plate, repeat the process, using a third plate.
Put the ore into a small cup. Soft plastic cups work nicely. The cup should be small enough that the ore fills it at least a third of the way up. Add some vegetable oil to the ore, and stir with a plastic spoon, or another nonferrous object, such as a popsickle stick. Keep adding oil until you get a thin black paste. Now gently place a strong magnet on the side of the cup. It should stick to the side as it attracts the ore. The ore should become quite stiff. Tip the cup over another cup to let excess oil and ore pour off. What remains in the first cup is our magnetorheological fluid.
We are now ready for the fun part. Hold the cup upright, and remove the magnet. Stir the liquid with the plastic spoon. It may be a little stiff at first, but will soon stir easily. Tip the cup a bit to the side, and bury the bowl of the spoon in the liquid. Now place the magnet on the side of the cup to stiffen the goop. The spoon will now stand upright when the cup is righted. The cup can even be inverted without losing any fluid, although a little oil may still drip out the first few times. Set the cup upright again, remove the magnet, and the solid mass slumps back into the cup, and the spoon falls over.
Put some fluid into a plastic bag, and let a magnet stick to the side of the bag. Now you can form the fluid into shapes by pressing the bag. The fluid will act like clay, and hold its shape. When you remove the magnet, the shapes slump into puddles.
Why does it do that??? Lets wait for the next part
