A little cheesy but.... "The x's and y's are in a relationship. In order for their relationship to function, work out, the x's must relation loyal to the y's. In order words, the x's cannot cheat on the y's. Look at this tables and tell me if their relationships will function..."
If I teach the vertical line test without having the students write the coordinates where their vertical line intercepts, they have a hard time understanding why it works. If they write the coordinates, they'll see that the x's repeat in the domain.