Please never give to beggars, they are often scams, and a significant number of human trafficking rings operate with beggars. You can take a child and force them to beg, because the trafficker knows that westerners are more likely to give to a child, than to an adult. Or even to someone who is disabled. In many parts of the developing world, there are plenty of people who have been physically harmed so they do better with begging (i.e., eyes burned, limbs cut off etc). You'd be horrified to know the truth.
And don't think that buying someone groceries is doing the right thing. I was once with a very well respected international charity in Mumbai, and I was talking to the worker about begging and as we sat in the cafe/corner store, a beggar came in with a westerner to buy groceries. They got all the essentials, including baby food, flour, salt, cooking oil etc. the westerner felt so good about it. Around 10 minutes later, the beggar came back, gave all the items back to the shop keeper, and they split the difference. The friend of mine, who had worked in downtown Mumbai (behind the Taj Hotel) said this was their most common scam, and was in partnership with the shop owner.
If you want to do something that has value and makes a real difference, donate to a charity that works there and is actively trying to change the situation. They know the reality of what is happening on the ground. And are often making real change to problems.
Westerners feed the begging culture, and often prevent children from going to school as its more profitable for the parents to send them out to beg rather than to get an education.
The other thing that always amuses me is the comments around charities trying to rip people off, and spending too much on administration. Yes, some charities are like this, but there are plenty of charities who are great stewards of their donors funds, and the people who are often the most critical of these charities are also the ones who will happily give money to a beggar that might be the victim of human trafficking or slave labor/debt bondage and there is absolutely no return on the investment - you will never change the situation of that child/person, and you would have been better to throw your money on the ground.