Several ant species herd aphids in much the same way that humans herd cattle and take their milk. Instead of milk, however, aphids excrete a sugary honeydew that the ants devour.
Ants go to great lengths to care for their aphids, often training them to defecate in a way that makes it easier for the ants to gather and eat the honeydew. In fact, the well-trained aphids will often withhold their honeydew until they are stroked and "milked" by ants.
Even more fascinating, ants will typically carry their aphids to new pasture lands and protect them from predators. In extreme cases, ants will clip off the wings of their "domesticated" aphids to prevent them from flying away when they mature.