Caustic soda, it is either beads or flakes.
I prefer the beads as there is a lot less dust, with the flakes there is way, way more dust that you don't want to get into your eyes, nose, mouth or on your skin as the moisture in the air can activate the caustic soda and you are now getting chemical burns.
It is really personal preference, that is up to you.
When you pour the beads into your liquid (never liquid onto the beads), pour the beads very slowly and don't mix up too much at a time and stir.
Depending on how much you mix, the liquid will get warm or hot or very hot so don't use thin plastic containers, use the thicker plastic containers, not metal.