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Money is not the only method.
Probably the money means more to them than it does to you.
I don't have any change
Some homeless people can feed themselves, they just can't afford rent. Therefore, you should not give money to all but only the starving and truly needy. Homeless people are a group, and work together to get through the day. Read source for further reading.
The person who is being forced money into their pockets may not want to receive charity. To force charity upon a person would be counter-intuitive
Homeless can be considered a flaw in our social/economic system. If such, giving money to homeless only temporarily fix the problem. This is unsustainable. The right thing to do is certainly to structurally fix the problem, and thus to give money to NGOs/politics that have the potential to do it.
They don't take credit cards.
if you give money to homeless people they will just continue on to use it on hookers and blow.