Weigh the person on a mass balance. Explain to your students that a person's mass, defined as the sum of that person's particles, is a property that will not change from planet to planet.(However, mass will change if they eat too much or go on a diet, etc.!).
Multiply the person's mass by the acceleration due to gravity, 9.8 m/s2, to obtain their weight on Earth.
Illustrate how weight changes among different planets by multiplying the acceleration due to gravity on other planets by the person's mass.