I am new to fruit & vegetable gardening. My watermelon vines (I have three plants) have been growing like crazy and I've been so proud of them! I've been propping up the ones on the ground on upside-down flower pots. One is growing really well and has the nice dark and light green stripes. However, today, I found three grapefruit-sized light green ones had split open. Only one of them was red inside - the others were white.
What happened? Can I prevent this on my others?

Usually splitting fruit means that it got too dry and then it was watered well and it plumped back up...thus causing a split. Try to maintain even moisture, but not too wet.