A married couple in your audience is having a  wedding anniversary and you want to  help them celebrate the special day with a little magic! 
     You borrow  their wedding rings and lock them together with a four-dial combination lock!  
     You hand the lock back to the couple and invite them to come up with a  "magic" combination number to open it. 
     For example, you have them add  their ages to the number of their children. 
     The lock didn't, so you ask them to try again,  this time asking them to use the numbers of their address (or whatever). 

     Finally, the magician asks them to dial in the year of  their wedding, say 1970. 
     They dial in the numbers 1, 9, 7, and 0 and the lock  pops open and the rings are returned.