Okay , i learnt this from a guy at work the other day and thought it was quite cool with the potential for trickery .(prerequisites are being able to showboat and the ability to add .)

It starts out with me and one other person . I ask the person to pick any 3 digit number . Lets say he picks 368 and get them to write it on the paper for all to see . Then you write a secret number on a seperate piece of paper , let noone see it , fold it over and place it infront of the person without letting them see it . In this case , the number can only be 2366 (Ill explain how you come up with this number later) . Then you get the person participating to write another 3 digit number under the first . Lets say they write the number 729 . Next , you write another number down under their number also 3 digit , and in this case it will always be 270(again , ill tell you how to find this number later). Then again you get the other person to write a 3 digit random number , its 485 this time , and in return you write under this one the number 514 (again ill tell you soon how to find this number) . So , infront of the person is a piece of paper with these numbers :

368
729
271
485
514

Next , you tell them to add these numbers up .

So if they can add they get this-

368
729
270
485
514
____
2366
So you tell them to open up the piece of paper with the one number in front of them , coincidentally , the exact number you wrote on the seperate piece of paper , which means you MUST HAVE READ THEIR MIND . Or did you . Heres the trick if you havent already realised .

The number 2366 - the first number 368 = 1998 . then look at then next 2 sets of numbers . 729 and 270 . The highest possible 3 digit number is 999 . Take 999 - 729 = 270 . Wow . Then the next two sets of numbers , 485 and 514 . 999 - 485 = 514 . Shock . So , the highest total you can get from the last four sets of numbers is , you guessed it 1998 . To get the number you "read from their mind" you merely have to add their first number with 1998 , then after they write each consecutive 3 digit number , just take their number away from 999 . And when they add it up , they will ALWAYS (if everyone added up okay) get the total .

Try it with different numbers on your freinds and try to wow them , its a great party trick .

(hope that all made sense )