That's mint. Off to the side, I boiled a few table spoons of honey and a cup of water and mint. Of course, once it's boiled down, the mint will turn brown. Remove the mint. Hopefully, it'll infuse with the honey water mixture. Mix it with the watermelon juice.
That's the juice pouring out. Juicer by Jack LaLane.
Add some lemon or lime to the mix, and there you have it. Add a tiny piece of fresh mint, and this is looking pretty good. If you have the fancy but sort of impractical ice cube trays by IKEA, then it's just that much cooler.