A lot of relationships are one way streets. Your students are probably facing that harsh reality in the form of unrequited love. They can buy bouquets of flowers and a heart-shaped boxes of chocolates, but it doesn't mean they'll get any back. Too bad they already ordered that enormous stuffed lovebug.
One-way relationships might be unavoidable in life, but math is much kinder (and your students should remember that). In mathematics, we can have relationships that are two-way streets. They're called inverse functions, and students should know how to find them.