Smell. Ants are extremely sensitive to smell and pheromones, and while you might not be able to tell, you give off plenty. Ants know immediately that you are a foreign *living* threat to their hill and they will attack you if just to try and make you go away.
Rocks and sticks will register as not being alive and rather something that they will have to move themselves or build around. They are usually familiar with natural occurences that may disrupt the colony, but once they stop moving, rocks and sticks aren’t threats to them.
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