Many 'science kits' and books for younger children include experiments that need alum, but they usually do not include the alum itself. We ordered 100 gr just to find out right away, that all experiments actually start with that quantity, and only one bottle will never be enough.
I teach a 4th grade science co-op each year and growing crystals is one of the students' favorite labs. Alum is the best for fast growing crystals. Salt will work, but the crystals grow more slowly and the saturation point is more tricky. (Don't bother with sugar unless you want to grow alot of mold.) This bottle gives me enough for 8-10 students to grow crystals in a jar hanging from a string. The crystals usually grow within a week and the students are really excited. Home Science Tools' Alum is consistant and has worked well each time. I can't always find Alum in grocery stores so I depend on getting what I need from Home Science Tools.