A ripe cranberry has four tiny pockets of air tucked inside its flesh. Those air pockets are what make cranberries float in water, and they’re also what make ripe ones bounce when you drop them. A really good cranberry will bounce about 10 centimetres off a hard floor.
Farmers actually use this to sort their harvest. The berries are poured down a series of wooden ramps with low wooden walls. Ripe, firm berries bounce over the walls and into a collecting bin. Soft, bruised, or rotten ones can’t bounce, so they just roll harmlessly down to the bottom and are thrown out.
The float trick is even more important at harvest time. Cranberries grow in low marshy fields called bogs. To pick them, farmers flood the bog with water. The ripe berries pop loose and float to the surface, where they can be scooped up in giant bright-red rafts. Almost no other fruit is harvested by flooding.
