How do ants find your lunch in the park?

By smell. An ant finds a crumb you dropped. It carries the crumb back to the nest. As it goes, it presses its abdomen to the ground. This leaves a smell trail. Back in the nest, the ant pokes the other ants. They run out and pick up the scent. By instinct, they follow the trail. Soon there are dozens of ants looking for more lunch crumbs.