I was riding the T this weekend in Boston, and had that quintessential mass transit experience. Even though the car was packed it was silent, with everyone minding their own business. The train inexplicably screeched to a halt in the middle of the tunnel (the Green line always screeches).

As our wait turned into minutes, a boy started asking mom questions. When he asked what building we were under now, a stranger got into the guessing game. Several in the car started eavesdropping on this small diversion, and then joining in to suggest landmarks on the streets above, each more obscure than the next.

It always seems special and rare to have a great exchange with complete strangers.

Bigger spectacles usually get top billing at a science festival. But I think the real magic starts when strangers get so comfortable they start cutting each other off in excited conversation. The evaluation data agrees: the top factor producing positive attitude and learning among festival attendees across all demographics? Face-to-face interaction with a science professional.

Engineering this isn't always easy. It took being trapped in a tunnel to get the T riders talking. But this is what the best street fairs, science cafes, and open houses do: they purposefully set the stage for real human interaction.