First appeared in Strange Horizons. Reprinted by Peter Straub's view of the new horror in Poe's Children. It was up for a Locus award.
Dewey Kerrigan is sent to live with the Gordons in Los Alamos. Like her father, Dr. Gordon is scientist who has a girl, Suze, her age. They have a picnic to watch the bright flash that has something to do with "the gadget," whatever that was.
They drive out to a green glass sea, for Suze's birthday present. They select pieces as souvenirs.