This game can be tailored to the particular child, and depending on how old they are you can just either ask them to put the organs in the right place, or talk about each one in a little more detail.

I usually do the organs in the following order (getting more difficult as the game progresses) and talk about/ask the child what each one is for:

HEART:
Ask them to look at the colour and guess what goes inside it.
Tell them that it acts as a pump, and is needed to push blood around the body.

Keen: could go into why blood is needed etc.
Ultra keen, try and grab a spare stethoscope and find their their heart beat at the appropriate place.

LUNGS:
Talk about breathing; try and get them to link up lungs to the mouth with the trachea (NB trachea does not currently exist as a component of the vest, but you can talk about it anyway!). Interesting fact: If the inner surface of the lungs could be stretched out flat, they would occupy an area of around 80 to 100 square meters – about the size of half of a tennis court!

DIGESTIVE SYSTEM:
Try and get them to link it all up, and explain what it does in terms of breaking your food down into smaller bits so it can be used to re-build things inside your body. Also talk about what happens in the end, and try and get them to guess what it comes out as (usually good for getting a giggle, but be warned sometimes the kids come up with very rude words much to the embarrassment of everyone around...)

KIDNEYS/ BLADDER:
Get them to link them up and talk about what happens when you drink too much water etc. Get them to try and guess what's in the bladder by its colour (also often gets another giggle).

LIVER:
Difficult one to explain. I usually go along the lines of it's like a big factory in your body where things are broken down and important things like stuff in your blood is made.
Ultra keen: talk about it also being needed to break things like poisons and alcohol, and try and mention what might happen to the liver for instance in alcohol poisoning.

INTERESTING FACTS / TIPS:
Small intestine is 7m long, but is coiled up to fit inside the body.
Furthermore, due to villi total surface area is 2000 square meters = half a football pitch.

By the time you turn 70, your heart will have beaten two-and-a-half billion times (figuring on an average of 70 beats per minute) = 175 million litres of blood = 50 Olympic swimming pools!