I saw this tiny cluster of aphids on a cottonwood leaf. At first, I thought I was looking at an egg cluster of something. But when the ants kept coming to check on them I took a closer look and discovered that in fact they were aphids. The ants tend these guys carefully because the aphids emit a sweet sugary substance(honeydew) that the ants love, so they protect their herd from predators and in return, get to harvest this sweet stuff!