Break the styrofoam cups into small pieces and place them into a blender. Add about 1 Tablespoon of water, cover, and blend on high for approximately 2 minutes. Remove the lid, scrape down the edges with a spatula and pulse it for 30 seconds at a time until it's completely ground up into small beads. (It won't be perfect, but that's okay).
Add 1 Tablespoon of Borax to 1/2 cup hot water and stir until dissolved and the water turns clear. Microwave for 20 seconds at a time to help it dissolve.
Add 1/4 cup white glue, 1/4 cup warm water and 10 drops food colouring to a large freezer bag. Squeeze and knead it through the bag until it's combined. (Make sure it's completely mixed before adding the borax).
Add the water and borax mixture to the bag and again, knead it until the water is absorbed and it turns into slime. (It's okay if you can't get all the water to absorb).
Add 2 cups of styrofoam beads to the bag, seal the bag, and then knead it until the slime and styrofoam are completely mixed.
Open the bag and allow to air out for 5 minutes before playing with it.