Question

Why melons and pumpkins crack while on the plant

Answer 1

Melons growing in fields ... Well, for one thing, not all the plants growing in a field get the same amount of water.  Some get too much, most get the right amount, and some get not enough.  Those that do not get enough water have smaller fruit, but their fruit is more sweet.  Those that get enough water grow the huge fruit, and they taste normal.  Those that get too much water grow large fruit, but it tastes watery.  Often times those fruits get so much water so fast that they literally burst, or rather crack open.
This bursting happens because the fruits do not transpire like leaves or stems or roots do.  They are  designed to retain water, like living balloons.  
Melons have two purposes.  Firstly, they are the cradle for seeds to grow and mature in.  Secondly, they are also storage organs for moisture.  This moisture  attracts animals that will eat the melon and the seeds, which are  designed to stay intact while passing through the digestive tract.  The  animal, after consuming the melon, proceeds to walk away somewhere and poop out the seeds.  Now the seeds have fertilizer to encourage growth as well as a new location in which to grow.