How a Venus flytrap counts to five without a brain

A Venus flytrap ignores the first touch. Brush one of its trigger hairs and nothing happens; brush a second within about half a minute and the jaws slam shut in a tenth of a second.

The plant has no brain, but it keeps count with calcium: each touch fires an electrical pulse that raises calcium in the leaf, and only two pulses close together push it over the line. If the prey keeps struggling, by about the fifth touch the trap seals and starts to digest.