The best questions to make you feel silly


Let’s start off with a straight forward question:

  • A bat and a ball cost £1.10 in total. The bat costs £1.00 more than the ball.
    How much does the ball cost?

“Pretty simple” I hear you say, “it must be 10p”.

Well, if you did answer 10p you would be in good company with more than 50% of students answering this question attending Harvard, MIT and Princeton and in other, less selective Universities, more than 80%.

This is the typically intuitive answer and many people jump straight to it. Unfortunately for our intuition, it is also wrong.

By now, some of you will have clicked off this page in disgust, some of you will be in disbelief and some of you will think, “of course 10p is not the right answer, I thought long and hard about it because your title primed me to think it was a trick question!” And a trick question it was.

It is one of the 3 questions which make-up what is called the “shortest intelligence test in the world” the Cognitive Reflection Test (CRT) – essentially a test designed to make you feel really silly.

I’ve recently been reading two books, both which brought up this question and spoke about our amazing ability as humans, even those who we perceive to be intelligent (e.g. more than half of those silly Harvard and Princeton chaps!), to get simple questions wrong. Our problem is that we have two methods of thought and processing stuff – one quick and one slow.

That question is designed to activate the quick part of our brain, coming to the snappy conclusion of 10p, but at that point, for some of us, the slow part of our brain takes over and goes, “woah, woah, woah! Hold on a minute! Let’s check this out before we look silly!”

And for others, the slow part goes, “yeah that sounds about right, let’s go with that, it makes a lot of sense, I’m just going to have a little nap and let you do all the thinking.” If you want more detail on this then you should read Thinking, Fast and Slow – it’s very interesting and probably much more ‘right’ than the way I’ve described it.


Anyway, by now, if you haven’t already gone back and worked it out, you must be itching for the answer to the first question, however instead of telling you I’d like you to answer the other two questions on the CRT. Try not to think too long and hard about them – try to answer them as if I hadn’t primed you. Use your logic and however you would answer normally, just don’t go, “he’s directly telling me this is a trick question, therefore I must look for the trick!”:

  • If it takes 5 machines 5 minutes to make 5 widgets, how long would it take
    100 machines to make 100 widgets?
  • In a lake, there is a patch of lily pads. Every day, the patch doubles in size.
    If it takes 48 days for the patch to cover the entire lake, how long would it
    take for the patch to cover half of the lake?

These questions are so interesting and fascinating because they are infuriatingly simple once you’re told the answer (although it took me a long time to be convinced of the answer to the machine question).

One other ‘really cool’ (my personal opinion) test they performed with the first question, was to print it smaller, in a different font, in light grey and in Itallics, and get another sample of students to answer it. This sample actually performed better!

The theory is: because the text was harder to read, students slowed down and took their time, they strained more and used more brain power, they activated the slow thinking part for the task of reading which inadvertently helped answer the question.


Sorry to be a super geek about all of this, but I think it’s very interesting. Thanks for bearing with me. And if you’re angry because you answered the questions incorrectly and feel silly, simply ask them to someone else, pass the silly bug on…

If you’ve got to this point and you haven’t thought long and hard about the answers then they are, in the appropriate order:

The ball costs 5p and the bat £1.05 making £1.10 together

It takes 100 machines 5 minutes to make 100 widgets – it takes 1 machine 5 minutes to make 1 widget, ergo…

The lake will be half covered in 47 days – double the half-a-lake and the whole lake is covered in 48