There are approximately 1.22 x 10^27 molecules in a 12-year-old’s body. Another way of saying this is that a 12-year-old has
[(one billion) times (one billion) times (one billion )] molecules – which is a humongous number!
If we imagined that each molecule was the size of a small sphere with a diameter of 1 cm (like a ball bearing), then:
One billion molecules would fill 2 big classrooms.
If we began piling (one billion times one billion) of these 1 cm diameter-size molecules over the entire city of Portland, the pile would rise to the height of Mt. Hood!
And if we wanted to pack (one billion times one billion times one billion) of these 1 cm diameter size molecules, we would need two spheres – one the size of Earth, the other the size of the Moon (and we would still have a few molecules left over)!
It is a good thing molecules are much, much, much, smaller than 1 cm!

This is how I estimated the number of molecules in a 12-year-old's body: Let us say that the 12-year old weighs about 45 kg (about 100 lbs). To make the calculation simple, let us say that the body is made up of only water (actually the body is only 2/3 water). One mole of water is 18 g. So, 45 kg of water will be [ (45 x 1000) / 18 ] moles. Each mole has Avogadro's number (6 x 10^23) molecules. So 45 kg will have [ (45 x 1000 x 6 x 10^23 ) / 18] molecules = 1.5 x 10^27 molecules. Since the body is only 2/3 water, we can guesstimate that there will be 1 x 10^27 molecules of water + the molecules in the remaining 1/3 of the body. Since these molecules are heavier than water, there will be fewer than 0.5 x 10^27 of them – let us say 0.2 x 10^27. This leads us to guesstimate that an average 12-year-old has about 1.2 x 10^27 molecules in his/her body.

There are approximately 1.22 x 10^27 molecules in a 12-year-old’s body. Another way of saying this is that a 12-year-old has

[(one billion) times (one billion) times (one billion )] molecules – which is a humongous number!

If we imagined that each molecule was the size of a small sphere with a diameter of 1 cm (like a ball bearing), then:

One billion molecules would fill 2 big classrooms.

If we began piling (one billion times one billion) of these 1 cm diameter-size molecules over the entire city of Portland, the pile would rise to the height of Mt. Hood!

And if we wanted to pack (one billion times one billion times one billion) of these 1 cm diameter size molecules, we would need two spheres – one the size of Earth, the other the size of the Moon (and we would still have a few molecules left over)!

It is a good thing molecules are much, much, much, smaller than 1 cm!

This is how I estimated the number of molecules in a 12-year-old's body: Let us say that the 12-year old weighs about 45 kg (about 100 lbs). To make the calculation simple, let us say that the body is made up of only water (actually the body is only 2/3 water). One mole of water is 18 g. So, 45 kg of water will be [ (45 x 1000) / 18 ] moles. Each mole has Avogadro's number (6 x 10^23) molecules. So 45 kg will have [ (45 x 1000 x 6 x 10^23 ) / 18] molecules = 1.5 x 10^27 molecules. Since the body is only 2/3 water, we can guesstimate that there will be 1 x 10^27 molecules of water + the molecules in the remaining 1/3 of the body. Since these molecules are heavier than water, there will be fewer than 0.5 x 10^27 of them – let us say 0.2 x 10^27. This leads us to guesstimate that an average 12-year-old has about 1.2 x 10^27 molecules in his/her body.