Many plants respond to outside stimulation, such as closing their leaves or flowers at night. Instead of nerves, the plants have cells that release chemicals in response to changes, such as changes in light. Those released chemicals cause some of the plant's cells to force water out, collapsing the cell. When the cells on one side of a leaf have high pressure, and the cells on the other side collapse, the leaf will bend towards the collapsed side, moving without muscles.

Sensitive plants take this further, responding not only to light, but also to touch and heat. They also release more of the stimulating chemicals, causing the reaction to happen fast enough for us to see.