Ripe cranberries bounce like rubber balls and float in water because they have air pockets inside them.

Cranberries have four distinct air-filled chambers inside their skin. These chambers take up a large part of the fruit's volume. When a cranberry hits a hard surface, the air inside compresses and then pushes back. This action works like a shock absorber and makes the berry jump into the air.In the early 1800s, a New Jersey farmer named John Webb noticed this trait. He saw that his firmest berries bounced down his stairs while soft ones did not. This led to the creation of bounce boards to separate good fruit from bad fruit. Today, fresh cranberries must bounce over a 4-inch wooden barrier to be sold in stores.If a cranberry is bruised or rotten, the air pockets break down. The fruit becomes soft and loses its ability to bounce. Instead of jumping, a damaged berry will hit the ground with a thud. Farmers use this to ensure only the best fruit reaches customers.The air pockets also help with harvesting. Most farmers flood their fields with 6 to 8 inches of water in the fall. They use machines to shake the berries off the vines. Because of the air inside, the cranberries float to the top of the water. This makes it easy for farmers to gather them in large groups.