You don't have to use a straw to do this (although watermelon juice is delicious and hydrating), but it's important to remove any extra water that emerges after you poke the holes. As you poke the holes, peer into the centre hole and watch for any juice that comes up. Whenever you see liquid, either quickly grab your straw or just tip the watermelon on its side and let it drain into the sink. Since watermelons are already so saturated with water, the more water you remove, the more vodka they can hold.