That’s a good question! I’m not too sure on the answer, but here are my thoughts. Crystals don’t ‘grow’ in the normal sense of the word – in other words, how we think of plants or animals growing. Instead, they grow because of the way they are structured. Crystals have what’s called a lattice structure – the molecules that they are made up from are all arranged in a very neat and ordered repeating pattern. This means that when other molecules come into contact with the surface of the crystal, in order to bond to the surface they can only join the crystal at specific points, and do so in such a way as to increase the size of the crystal. Over time, this causes the crystal to increase in size, or appear to ‘grow’. When you get ‘grow your own’ crystal packets, I think you have to dissolve the contents in liquid. This means that all of the molecules that can form crystals can move around and join up with each other. These bigger pieces move around and connect with other pieces, until eventually the piece is big enough to fall out of the solution (which is called crystalisation). There will still be molecules in the solution that can make crystals though, so they will continue to bond with the main crystal (making it look like it’s growing) until no more bonds can be formed.