Things You'll Need
How to Do the Magic Envelope Trick. Who knew you could read minds. This is one of the pick-a-number tricks, of course. They may be a little suspicious when they all come up with the same number. This trick can make a great brain teaser for the young and old alike. Read on to learn how to do the magic envelope trick.
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).
The quicker you give the instructions for this, the less time audience members have time to think about the trick. If an audience member says you're wrong, tell them they should bring their calculations to you. Go over each date with them until you have them admitting they made the mistake. This is a great way to start out a math lecture or take a little break from a long conference day. Using a volunteer from the audience to write down his numbers is a way to keep them from realizing they have come up with the same answer. Hide this fact or the audience will recognize the trick as a math teaser.