Cells in the epithelium hold on to each other like people in a crowd. In a typical crowd, everyone stands packed together, and it behaves like a solid. But if people start pushing through, the crowd can flow. Cells have four ways to set up such pushing.

The first way: each cell pushes randomly, paying no attention to its neighbors. The second: cells agree to move in one direction, like a group of people heading for an exit. The third: cells chaotically contract and expand, shoving their neighbors. The fourth: if a cell is stretched, it contracts in response, and this contraction propagates onward like a wave.

The most reliable indicator is the coordination of movement between neighboring cells. If cells move each on their own, coordination is low. If they pull each other along, it is high. If they move toward each other, coordination becomes negative. Based on this feature, one can unerringly determine which of the four mechanisms is operating in the tissue.

Previously, scientists had to guess the mechanism or use complex experiments. Now it suffices to record a video of the tissue under a microscope and calculate how the cells move. It's like figuring out from a recording of a crowd why it started to flow: from random jostling or from a collective command.