Dawn and her roommate had come up with a system.
They spent a lot of time together, by definition; at least eight hours daily if not more the way their classes and social circles overlapped. Personal secrets were guaranteed to come out eventually, no matter how hard they tried to keep them back. So:
One truth given for every truth revealed, of equal seriousness.
Buffy would probably kick Dawn's ass for agreeing. But Dawn had known the moment she met Cassie Frasier that the other girl's background was just as different from the pampered upper middle class kids around them as hers was. Maybe not Sunnydale level strangeness; maybe just human trauma, given the fact that her Moms were both Air Force, and that one had died in the line of duty. But it was enough for her to trust Cassie was serious when she proposed the deal.
Still, she'd figured she'd just end up sharing a few bizarre but innocuous details if it ever came to that, like the fact that her sister's first boyfriend had a mental break once and killed her sister's best friend's goldfish.
...Until she caught Cassie moving a pencil one day with her mind.
Cassie stared at her for a long moment, then said: "I was genetically tampered with. I'm-- not Canadian."
And I'm scared to death of what that might mean, her expression added.
Dawn froze, heart pounding in sympathetic recognition, then made a decision: "I was made to order too," she replied, softly. "Someone tried to kill me once because of it."
Then she lifted her shirt, exposing her scars, with a grimace.
Three truths for three: a cord of three strands is not quickly broken.
In that moment Dawn knew she and Cassie would be best friends from that day onward.