Check for asthma. If child is asthmatic, check with parents about severity, and if the child can do five minutes of fairly heavy-impact exercise. If they can do it, ensure they have an inhaler, and ensure they don't push themselves too far.

Take child's pulse (radial side of forearm), and write it down.

Make child run on the spot 3 mins and then do starjumps for 3 mins or any other combination of the above.

Check pulse again.

There should be a difference. If not, it's an anomaly, and you can make them do the experiment again.

If you're doing this experiment by taking the radial pulse, you need to start by asking questions (like "Have you had your pulse taken before/seen it done on TV?" and "Why do doctors take your pulse?" to make sure they know that your pulse reflects your heart rate, and if they don't, make sure to explain it before carrying on!

So, why does your heart rate go up?

What is the heart for?
-They will usually say say pumping blood around the body/beating. My favourite question after this is "What is blood for?", to which the answer is always "It keeps you alive". You can then progress from here by asking why (which they often will not know, even in surprisingly old children), and then explaining why we need blood. I usually go along the lines of "What do we need to do to stay alive?", get them to work out that we need to breathe, eat and drink, then think about where air and food go into our bodies, and then how might we be able to transport these things from your lungs/stomach to wherever they're needed, like your muscles to jump around or your brain to think - and then you have ended up working out what blood does! (You can use the analogy of lots of lorries (blood cells) driving along roads (blood vessels) carrying cargo (oxygen/food). You can go further into the idea of cells needing energy from glucose and oxygen with older/interested kids.

- With older/interested children you can then develop the idea by talking about the heart as a double pump and the pulmonary and systemic circulatory systems. This works well if you've got the heart model to hand (on tour), or if someone else has just explained the heart model to them!

-Then you can talk about how your muscles are more active when you're doing exercise so they need more 'fuel' (oxygen and glucose), therefore you need to get more blood to them in a shorter space of time, therefore the heart beats faster!

-You can also talk about breathing rate (are they panting?) and recovery times and fitness if you wish to. You need to breathe at a higher rate and more deeply to get more oxygen into your lungs, and then move it to the muscles more quickly, because they need more energy and so are using up oxygen faster.

(N.B. I find this experiment is quite useful with groups of lively children, as provided you can get them to be reasonably sensible and you have enough space, they can jump up and down until they're quite tired and then they will often sit down calmly and listen to the explanation afterwards!)