I was SO excited to learn about these last year. You can purchase them on Amazon & from online specialty kitchen stores. They're silicone & flexible, so you can even squish them into a lunch box, if you need too. (Just tell your kid/hubby/whoever to put the lid back on afterward, so it doesn't leak the left over after they're finished eating)
All you do is fill them up *almost* to the top, & stick 'em in the freezer. (Leave about 1/2" space at the top, so that it doesn't leak when it expands from freezing)
Here are the different things we pour into them:
Soft homemade ice cream (from our Blendtec)
Flavored water with fruit chunks
Chocolate milk (see my healthy white choc. milk recipe HERE)
*These are GREAT for my son who doesn't like vegetables. He loves green smoothies when they're frozen in these little push pops. I make sure he has at least one every day!