Normally, I use a knife... But I usually end up making the slices way too small. So here's a new way to open a watermelon (only for people who live in a building of at least five floors).
1: Buy a watermelon (should be a nice big, juicy one)
2: Climb to the top floor of your building
3: Find a balcony over the street (windows work too)
4: cut a hole in the watermelon (with a knife, or whatever you have)
5: Fill the hole with something (colorful, sticky liquids work best), then cover it with tape or something... Something secure so it won't fall out.
6: Throw the watermelon out the window/balcony as hard as possible, as close to the sidewalk as you can
7: If done successfully, you will have splashed some innocent people on the street with watermelon and whatever you put inside it!
8: Plead to the police (if they come), and blame your nextdoor neighbor!
If you don't want to do all that, that's OK... Another cool way to open a watermelon is with this!