Problem: You owe your roommate for toilet paper, but you don't have any cash on hand.
Solution 1: You add the money to the running total you keep on a whiteboard or random spreadsheet and plan to buy TP next time.
Solution 2: You run to an ATM, get your $20, and hope your roommate has change. She doesn't, because she doesn't carry cash either. You revert to Solution 1.
Solution 3: Just always carry cash. (HAH!)
THE REAL SOLUTION: Use SquareCash.
I love SquareCash. It's the best thing since Undo Send. It's a service that lets you email money to people. When you sign up, you link your debit card to your email address. When you want to send somebody money, you send them an email with a dollar amount in the subject line, and put firstname.lastname@example.org in the cc field. The person you're sending money to links up their debit card, and the money is transferred from your checking account to their checking account. There are no fees ... get that? NO FEES.
Every Thursday* is "Favorite Things" day, where I bring you a short recommendation of a product or service that I have found useful and/or helpful. Unless I say something, I'm not paid or compensated in any way for the plug. I just do it out of the goodness of my heart.