Carnivorous plants such as the Venus flytrap depend on meals of insects to survive in nutrient-poor soil.
They sense the arrival of juicy insects, lured by the plants' fruity scent, with the aid of sensitive trigger hairs on the inner surfaces of their traps.
Researchers reporting in the Cell Press journal Current Biology looked more closely at exactly how the plants decide when to keep their traps shut and begin producing their acidic, prey-decomposing cocktail of enzymes.
The short answer is: they count.