Ants will not cross a chalk line

Chalk is actually made up of calcium carbonate, which is basically broken-up seashells. Ants are not big fans of calcium carbonate, because they find that the chalk interferes with their ability to follow the scent trails of other ants. Ants rely almost completely on scents to find food, water, and even their way back to their nest. Without scent, ants are basically completely blind and will easily become lost and not find their way back to the colony.