Promises of emotional or sexual character made by unmistakeable eye contact only. The lack of spoken words often leads to misinterpretations with two very different conclusions by the two sexes. Probably with less confusion between same-sexed people.
In the stereotypical scenario, a man and a woman catch each other's eyes at a party. He concludes that she's committing to some serious monkey business and searches his wallet for a condom. She on the other hand feels very special and pretty and hopes to be asked out on a date. By the end of the evening at least one of them will be breaking their eye contract.