Ferrofluid is a liquid that reacts to magnets when present because it's made with iron oxide/particles that become suspended in liquid (oil). With magnets, you can manipulate the shape of the particles. Basically, one of the science teachers gave me some iron oxide and my students and I have been trying to find new and interesting ways to use it and then writing about what we did.
Step 1: BoM
Container with lid
Hot glue (to seal the cap)
Step 2: Oil
Start with adding oil to your container, any kind of oil will work. I used super old vegetable oil that's been sitting in my classroom for years.
Step 3: Iron
Add about 1/2-1 tsp of iron oxide (depending on the size of your container). Put the cap on the container and your ferrofluid is ready to play with. You can use hot glue to seal the lid.
Step 4: Less Is More
Less is more, if you put too much in like I did, then the mixture seems to become very muddled. Made worse by students who insist on shaking things. If your fluid is shaken too much and turns all black, let it sit undisturbed for a few hours or days and then come back to it.
Stronger magnets will yield cooler results, all we have to work with are magnets for holding up papers.