Explanation: Imagine that you see a money on the ground. When you go pick it up, it disappears. It was too good to be true. What happened is that someone had a piece of string on the bill and when you went to pick it up, the person pulled on the string so you could get it. So when a string is attached to something, it must be a trick of some sort that requires you to do additional things to get what you want. Because tricking people was popular, the phrase "no strings attached" because common by salesmen to indicate that they are not tricking you to buy something.
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