Australian science writer Margaret Wertheim got together with her artist sister to crochet a coral reef to draw attention to the endangered Great Barrier Reef. The project visits New York this month. She’s joined by local participants Barbara Van Elsen, founder of the NYC Crochet Guild, and Njoya Angrum, founder of the Harlem Knitting Circle to talk about physics, crocheting, global warming and grassroots needlework.