To blow someone’s mind, just take a paper from your pocket showing the same number as you have just found.
Now, the question is how to do it? While writing the number beneath the number written by someone else, make sure the resultant is 999999. So, the number will be 2*999999 + the number on the extreme top which you have written. You will be having matching results!
Then, ask them to subtract the year they were born in from the final resultant. Like for an example assume it’s 2000, it would give you 814. Then, look closely at the number in the last, the first digit would indicate the shoe size and the last two digits would give the age.
If the addition of the two digits gives you a two digit number, carry over and add it to the first digit to get the result.
If the square is a 2-digit number, append its last digit and carry the first digit to the last digit of the product. And you’re done!