Ask for a volunteer from the audience. Pick someone who is noticeable, such as a person with a red shirt or distinguishable features.
Multiply the current year by two (example--2000 x 2 = 4000). Write this number down on a piece of paper and seal it in an envelope.
Ask each audience member to write down their year of birth. Under that number, they should write the year in their life that something important happened, such as the first day of school, college graduation or the date of their wedding anniversary.
Tell the audience to write their age next. Each number should be written in list, on a piece of paper.
Have each person write down the number of years that have passed since the important event in their life.
Instruct the audience members to add all of these numbers up. Their final number should equal the secret number you have written in the envelope (example - 1990 + 1995 + 10 + 5 = 4000).