You may think all penny-squishing machines are alike. They all take your 51 cents and return to you a flattened copper oval, and they're all located at tourist traps. How different can they be? For collectors, it's a matter of deep importance. Is the contraption a rare hand-crank model, or the more common and less exciting automatic type? Is the design an old, good one, or a newer commercial-looking thing advertising a sports team? Regardless, the penny itself should be of sentimental value to its owner. Ideally, it's a small work of art. To this end we recommend the Laffing Sal penny stretcher at Fisherman's Wharf's Mus?e M?canique. Although it is a motor-driven device and the name is spelled incorrectly on the coin (it says "Laughing Sal"), the picture on it is bizarre and funny, like Laffing Sal herself. It's a good likeness. Besides — this may be a bit of penny-collector esoterica — the machine seems at home near its coin-operated brethren in the museum, which we feel results in a superior product.