Joel Pomerantz has a lot of nerve asking people to think and walk at the same time. He also has a lot of nerd. In fact, he bills his ThinkWalks — designed especially for locals — as "nerdy tours for San Franciscans." Possibly the man who coined "the Wiggle" to describe the city's best cross-town bike corridor, Pomerantz offers sliding-scale strolls that explore what makes San Francisco such an unusual city. He thinks it's because these parts boast a headlong collision of the urban and the natural — like the buried waterway under the Wiggle and the Mission District's latent ecology. So buy the ticket and take the stride: fresh geography, stronger legs, and opened eyes ahead.

(415) 505-8255, www.thinkwalks.org