As we're all taught as children, the traditional response to "thank you" is "you're welcome." This reply goes back to the early 20th century, but since that time a slew of hip, laidback alternatives—"it's nothing," "forget it," "think nothing of it," "my pleasure," "no sweat," "no worries," among others—have enlivened the language of politeness. One, however, isn't always welcomed with open arms, and we don't know why exactly. That oft-maligned expression is "no problem."