Even before they can talk, young babies know that two people must have a close relationship if they're willing to do to anything that involves swapping saliva.

Kissing on the mouth, sharing a spoon, taking licks off of someone's ice-cream cone — all of these activities generally only happen when people have an especially intimate relationship, and this fact appears to be obvious to infants who are only 8 to 10 months old, according to a new study in the journal Science.

"From a really young age, without much experience at all with these things, infants are able to understand not only who is connected but how they are connected," says Ashley Thomas of MIT, who studies what babies and young children understand about the complexities of their social world. "They are able to distinguish between different kinds of cooperative relationships."

Who do babies look to first?

Thomas and her colleagues reached that conclusion after showing videos of carefully crafted puppet shows to babies and toddlers.

One of their videos shows a woman rolling a ball back and forth with a blue fuzzy puppet. Then another woman shares an orange with that same puppet by putting a slice of orange in her mouth, then letting the puppet nibble on the slice, and then putting it back in her own mouth...

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