I've usually got some spare change. Most of us have. But if a stranger asks us for it, our definition of "spare" suddenly becomes complex, extending well beyond economics and thrusting deep into sociological-thesis territory. Our assessment is affected by so many things, including our recent history of charitable giving, the accessibility of our purse or pocket and how much we loathe mankind that particular day. The disadvantage we incur in giving some money is usually eclipsed by the benefit to the recipient – we're probably talking a meal costing a fraction of the amount commuters splurge daily on car resprays, depilation treatment or mojitos. But we tend to take a hardline stance, refusing to judge the spareness of our change, what with unforeseen expenses stacking up Jenga-like over the next decade in the form of nappies, tax demands and Ryanair baggage charges.

My friend Tommy was travelling on the Tube last week when a homeless man gave a speech to the carriage, explaining that he was looking to buy food and wondering whether anyone had any spare change. He then walked down the aisle to total silence and complete indifference – until he reached Tommy, who decided that yes, some of the cash in his pocket was surplus to requirements. He pulled out some change, removed some pound coins for his own dinner, and put the remaining £1.50 or so into the guy's cup.

"Are you taking the piss?" the man said. Tommy assured him that he wasn't. "Are you taking the piss?" he repeated, clearly livid that Tommy hadn't handed over the pound coins too. "Er, no," Tommy said, uneasily, wondering how much more angry this guy would get if he found out about the savings account he had with his wife. "Hold out your hands," the man said. Tommy refused. "I said hold them out," he repeated. When Tommy failed to do so, the guy emptied his cup full of change over Tommy's head and stormed off the train. Tommy sat there, stunned; he was the only person to have displayed any generosity, but was now having to deal with abject humiliation. As he got up to leave at the next stop and the coins rattled to the floor, his fellow passengers shifted uncomfortably, the Spare Change Question having just gained an additional, unwanted layer of complexity.