Conductors on San Francisco’s beloved cable cars ring bells to signal the gripmen, whose muscular tugs on the cable-grabbing levers start and stop the cars on the city’s hills. Two clangs: OK to go. Three clangs: Stop immediately. Lots of rapid clangs: Pretty girl ahead. Replicas of the signal bells (11.5 inches top to bottom, $70) are sold at the Cable Car Museum shop. Admission is free. 1201 Mason St., San Francisco, (415) 474-1887, www.cablecarmuseum.org.