Before you start you need to make two sets of at least 15 cards. On the cards in the first set use several different coloured pens - one for each card - to write the name of the colour you have used. For example, write "red" with a red pen. You can repeat the colours if necessary.
On the cards in the second set write the names of colours that are not the colour of the pen. For example, write "red" with a green pen, and "blue" with a black pen. Ask the children how quickly they can tell you the colours of the words on the first set of cards. Make a note of this on a score sheet.
Using a stopwatch, ask a pupil to time how long it takes for the children to tell you the colours of all the words in the first set. Record this on the score sheet.
Now ask them how long they think it will take to tell you the colours of the words on the second set of cards. Repeat the experiment and write the time on the score sheet.
You should find that it takes longer for them to tell you the colours of the words when there is a mismatch between the colour and the word.
Depending on the age of the children you may find variations in the time differential.
You could try the experiment with younger children who haven't learned to read, or you could split the class into small teams and compare results.
The effect you are measuring is called the "Stroop Effect", named after John Ridley Stroop, who first noticed the effect in the 1930s. It seems to stem from our ability to recognise words being quicker than our ability to recognise colours. There is a mismatch in the information our brain receives that causes us to stumble over the words.
Sophie Duncan is project manager for science at the BBC