Have one member of your group be the Sun, one member Mercury, and one member Earth. Have Mercury move about the Sun in a circle and Earth in a bigger circle. Have Mercury complete an orbit in about half the time Earth completes an orbit (this isn't actually the correct ratio of orbital periods). Ask Earth to verify that Mercury's angular distance from the Sun (its elongation) increases and decreases. Have Earth call out when Mercury's elongation is greatest. The rest of the group should note where in their orbits both Mercury and Earth are when this happens and how many orbits each has completed by the next time greatest elongation occurs. Have the entire group discuss the relationship between the interval of time between successive greatest elongations of Mercury and the synodic period of Mercury.