They're two-piece molds; here's how they work:
To freeze: Fill bottom half of mold up to seam. Put top half of mold on (match notches). When frozen, remove from freezer. WITHOUT removing the top, fill the rest of the way (but leave a little space) and put back in freezer. Because of the magic of water molecules (and how they expand when frozen), this method prevents the mold from leaking.
Thanks for reading!