Fellow blogger Daniel Hamermesh recently explained the virtues of exchange as a painter helped him break into his Berlin apartment. My exchange example is not as glamorous. Shopping at the local co-op in Cambridge, I heard over the public-address system, “If you are the owner of a gray Subaru Outback, you are being towed!” I leaped over a low chain and made a break for the parking lot, as a mother nearby offered to watch my daughters (ages 1 and 4). The Subaru was hooked up and about to be hoisted onto the tow truck. In view of my timely arrival, the tow-truck operator offered two options: Pick up the car later that day in Somerville for $200, or pay $50 (cash) and he’d unhook the car now. An offer I couldn’t refuse. Everybody gained, yet I am still furious!