Giant Corn was apparently common in a valley inhabited by Ground Whales which was about a day's walk away from the Croods old cave. Giant cobs could be found growing from the ground and apparently fire would cause their stem to explode and make the corn blast off like a rocket which upon impact would explode into giant popcorn.
In the Movie
After the Croods began fooling around with Guy's fire, it caused some of the giant corn nearby to ignite and blast the Croods, Guy and Belt into a large tree upon which it exploded into popcorn. However Grug told everyone not to eat it since it was "new".