Why do ants walk in lines?

Ants line up because a foraging ant lays down a pheromone path after finding food. Other ants smell that chemical track with their antennae and follow it. Each ant that travels the route adds more scent, so the useful path becomes easier and easier to find.

The clever part is that the trail can vanish. When the food is gone, fewer ants walk there, so the scent stops being renewed and fades away. The colony can change course without a boss, simply by trusting the strongest, freshest smell underfoot.