They are made of silicon and come stuck to a sheet of thick plastic. When you're done, you just stick them back on to be used again and again.
Our glasses seemed to take on their own personalities and it was impossible to grab the wrong one. I used to think that drink markers were frivolous, but now I consider them essential. So much easier than reading lip prints!
Coco didn't get a drink marker on her sippy cup, but she was delighted to take part in the toasting. Cheers! ;) Do you use drink markers? What are your favorite ones to use?