Try this

[Derek McMillan]

This is referred to as a chestnut but it amuses me.

x = 1

Therefore: x² = x

x² - 1 = x -1

Factorising x² - 1 we get

(x - 1)(x + 1) = x - 1

Dividing both sides by (x - 1) we get:

x + 1 = 1

Substituting 1 for x we get:

2 = 1

Try this on a year 9 class last thing on a Friday afternoon.

I dare you.