ver wondered why, in a bag of popcorn bought at the movies, there are always some unpopped kernels at the bottom?

Well, a Purdue University team led by food chemist Bruce Hamaker of the Whistler Centre of Carbohydrate Research has the answer.

"It is a matter of scientific curiosity and we are in the food business, so we decided to find out," Purdue's Rengaswami Chandrasekaran said.

After analysing 14 varieties of corn popped in a microwave oven, the team discovered that the key pop-ability factor is the kernel's cellulose hull, known as the pericarp.

Chandrasekaran, a crystallographer, found that the pericarp in corn that popped has a stronger crystalline structure than the pericarp in those that didn't.

"With a better organised crystal structure," he said, "the kernel retains moisture better. It swells as it heats up until it finally explodes, creating popcorn."

If the moisture leaks out prematurely or the pericarp collapses, the pressure will not build and the frustrated consumer is left with unpopped kernels that stick in your throat, plug up your teeth and dislocate your dental fillings.