How does your brain tell you to stop if you're crossing the road and you have to stop quickly? - Ruby, aged nine, Rochester, UK

The human brain is really clever. It keeps our heart beating, it allows us to see and hear the world around us, and it also helps us to make hundreds of decisions every day.

Sometimes decision making is hard, like deciding what book we want to read next. At other times decisions seem to happen without us even thinking, like when we stop crossing the road suddenly if a car is coming. So, how does our brain make decisions?

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Our brain has two ways of thinking: slow thinking and fast thinking. We use our slow thinking when we have to do something difficult, like our homework.

Our slow brain helps us to think through things logically, and it helps us to find the best answers. It also helps us to plan for the future, like thinking about our weekend activities or what we want to be when we grow up. Our slow brain is what we tend to think about when we think about our thinking.

The other way our brain makes decisions is by using fast thinking. We use our fast brain when we perform a task without even thinking, like when we jump out the way of a ball thrown towards us, stop suddenly when a car is coming, or sing along to our favourite song without reading the words.

We also use our fast brain when we know the answer to a question automatically. What is two plus two? What colour is the sky? Is chocolate delicious? We know these answers straight away. Our fast brain helps us to answer questions like these quickly and easily.

Working together

What's even more interesting is that our fast and slow brains work together. When we learn a new task, like playing the piano, we have to use our slow brain at the beginning. We have to concentrate in order to learn how to read music, and we have to try and memorise which piano key plays which note.

But over time, when we practice, our fast brain starts to take over from our slow brain. This means that eventually we are able to play some songs on the piano without even thinking.

Sometimes our fast brain makes mistakes though, like when we wave at someone by mistake because they look like our friend. Our fast brain mistakenly recognises that person because they have the same hair or clothes, and it accidentally tells us it's our friend without thinking.

This can be embarrassing, but our fast brain is only trying to help. If we used our slow brain all the time, we would never get anything done as it would take too long.

So next time you make a decision, have a think about whether you used your fast or your slow brain. Did you have to think things through? In that case you used your slow brain. Or did you know the answer straight away? If so, you used your fast brain!

And, remember, whenever you are struggling to learn something new, it's only going to be difficult for a very short time. If you keep practising it won't be long until you can do it without thinking. Your slow brain is doing the hard work right now, but your fast brain can't wait to take over.

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Author: Nicola Power - Lecturer in Psychology, Lancaster University