people asking for money.... what do you do?

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today I was on my way to work and in the metro there was a man in a wheel chair. he came up to my girlfriend and I and he said he was a victim of Katrina and that him and his daughter needed $23 dollars to keep their hotel room or else they would be kicked out. He asked for some money. I only had a 20$ bill on me and I told him I didn't have any small bills. I didn't see his daughter anywhere, but do you think this was a scam? If it wasn't I kinda feel bad but was I supposed to give him my $20 bill? have any of you guys been asked for money from people? what do you usually do? I usually feel bad if I say no, but I never know if these guys are truthful or just people looking for money to buy booze or drugs. Thanks!