The setup to this trick is to tell the class the story about Tommy while writing it out, numerically, on the chalkboard. Tommy did not like math; so when his teacher asked him to write out his multiples of 9 through 9 x 10, Tommy was not too happy. He knew the first one, 9, and wrote it down. By way of stalling, he decided to count out how many more multiples of 9 he had left to do. Since he had one done, he knew he had 9 more to go and decided to number the rows 1 through 9 below his first answer. Stalling further, he decided to double-check his numbering and renumbered the rows, this time beginning from the bottom row -- where he had finished his numbering with 9 -- then going up. At this point the teacher walked by and praised him for finishing his work so quickly.
Give three students calculators and bet them that you can add a list of five-digit numbers faster by hand than they can on a calculator. To make it "fair," have each student help create the math problem. Have the first student write a five-digit number on the chalkboard then tell the second student to write a second five-digit number below the first. You write the third number, being careful to make each digit add up to 9 when added to the number above it. The last student writes the fourth five-digit number; then you write the final number, using the same method as before. Have the students begin adding the numbers with their calculators. Pretend to add the numbers from right to left. To solve, deduct 2 from the final digit of the first five-digit number and put 2 in front of it. For example, if the first five-digit number chosen was 26,374, the answer will be 226,372.
Tell the class that they are going to use math to "calculate" their date of birth. Depending upon their math skill level, have them use calculators or calculate manually. Tell them to begin by adding 18 to their month of birth, multiply the result by 25, subtract 333, multiply by 8, subtract 554, divide by 2, add their birth date (day of the month), multiply by 5, add 692, multiply by 20, add the last two digits of their year of birth then subtract 32,940. The result will be their date of birth in mmddyy format.
Ask your students to think of a number between 1 and 10 and keep it to themselves. Next have them multiply their number by 9. If their result is two digits, have them add the two together. Then have them subtract 5 from their result. Now tell them to count as many letters in the alphabet as their result; for example, "1 is A, 2 is B, 3 is C." After they have gotten to their letter, ask them to think of a country that starts with their letter. Most will think "Denmark" since everyone will be on letter D. Tell them to think of the second letter of their country and think of an animal that begins with that letter. Most will think "elephant." Without explanation, exclaim, "There are no elephants in Denmark!"