Everybody reading this right now can put their hand through glass, it's just that in every case we'd break the glass, and we'd probably injure ourselves in the process. How is it that magicians can do it without damaging either themselves or the glass?
The answer is that they don't, they just cheat. A mirror panel at the back of the prop hides two smaller glass panels, one of which is simply pushed out of the way to allow the magician to put their hand through the space.