"Shit. My Fitbit's stopped working. Is it the battery?" Tilda clicks with annoyance. "How many steps have we done?" She bangs her Fitbit. "It doesn't count unless it goes on my Fitbit. I might as well not have bothered." Tilda's Fitbit is her latest obsession. For a while it was Instagram, and our daily walk was punctuated by her taking endless photos of raindrops on leaves. Now it's steps."