How does a girl get a free drink at a bar?

When girls go out with their friends drinking, what makes a guy want to buy her/them a drink? I know there’s the obvious (being hot, thinking they might get laid etc)... My girls and I want to get free drinks without being too pushy and rude. GUYS any tips on how to go about this at the bars?
How does a girl get a free drink at a bar?
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