You need a white T-shirt, permanent markers, isopropyl alcohol (we used 70% though 90% would be fine too,) a dropper bottle (we used a non-medical syringe,) a large plastic cup, and a rubber band. Here you see the boys posing with their new T-shirts ready to start the project.
Slip the plastic cup inside the T-shirt so the opening of the cup faces towards the front panel of the shirt. On the outside of the shirt, pull the shirt tightly over the cup and put the rubber band around the top to secure it.
Put spots of various colors on the section of the shirt secured to the cup. You can put wavy lines, too, if you like the effect.
Because the syringes are graduated, the boys made up math problems as they went along, like, "Mom, if I had 30 mm of alcohol to start and now I have 10 mm, how much did I use?"
We did have a PROBLEM though. I rinsed the shirts in water before washing them, but all the ink washed away. Next time I am going to rinse them well again, then run them through the dryer to evaporate off all the alcohol (it is very volatile, which is why it has such a strong odor) before washing it. The boys have been bugging me to get more shirts ever since.