1. Mystery square
Look at the two triangles below. The four shapes from the top triangle have been rearranged to make up the bottom triangle.
But now the triangle at the bottom has an square free. How can this be?
2. Count the 'F's
A simple one - read the following sentence and count how many 'F's you see. Concentrate and read it slowly - but do it just once before deciding on your answer.
FINISHED FILES ARE THE RE-
SULT OF YEARS OF SCIENTIF-
IC STUDY COMBINED WITH
THE EXPERIENCE OF YEARS.
3. Win a car!
Imagine you're on a gameshow. You've reached the final game and have to pick between three doors, labelled one, two and three. One door has a car behind it which you can win if you pick correctly. The other two are empty - pick them and you'll go away with nothing.
You decide to pick door one. But instead of opening it, the gameshow host opens door three and shows you that the car wasn't behind that one. He then asks if you want to stick with your choice of door one, or change your mind and go for door two.
Do you stick or switch? Does it make a difference?
4. Medical testing
Suppose that there’s a disease which affects 1 in 10,000 people. It has no obvious symptoms so people need to take a test to see if they have it. The test correctly identifies whether someone has the disease 99% of the time, but gives a false positive the other 1% of the time.
If I take the test and it's positive, what’s the probability that I actually have the disease?