It spins. It stirs. It's attracted to metal. It's a stir bar.
It's the piece you need to make your stir-plate function.
Here are some products you will want to add for use with your stir bar:
5 Gallon Glass Carboy
6 Gallon Glass Carboy
6 Gallon PET Carboy
6.5 gallon bucket
6.5 gallon bucket drilled for spigot