Transcript:

Yesterday, we looked at the number of atoms in our body compared to the number of visible stars in the universe.

So it's not so easy to count a lot of small things. It's very difficult to count the number of atoms in your body, but you can do it using scaling rules. And the most important scaling rule to remember is that one mole of anything contains Avogadro's number of particles or units. 6.023 × 10²³. This many number is present in one mole of anything.

Now it becomes easy to count lots of small things.

But you have to be careful with small numbers because it can give a completely different picture.

For example, if you count the number of non-human cells in your body, like all the bacteria, all the mites, all of this stuff — if you count the number of non-human cells, the number of non-human cells are more than the number of human cells.

Wait a minute, how is that happening? But if you calculate the mass of all the non-human cells, it is miniscule. The mass of the human cells is far more than the mass of the non-human cells.

So, small numbers are tricky.