Imagine you're holding on to someone's mobile phone for a bit. Now forget that. Pretend you have to go home, and halfway to the train station you realise you have two mobile phones in your pockets. Realise that you own only one. Go back and return the phone. By doing that, make sure you miss the train. Now go and pay half a fortune for a cab. Remember that a train costs 15 times as little for you.