ction and go a
different direction. The proton has the larger inertia. It is the
electron which is the easier to start or stop.

How else do they differ? They differ in their actions. Protons don't
like to associate with other protons but take quite keenly to electrons.
And electrons--they go with protons but they won't associate with each
other. An electron always likes to be close to a proton. Two is company
when one is an electron and the other a proton but three is a crowd
always.

It doesn't make any difference to a proton what electron it is keeping
company with provided only it is an electron and not another proton. All
electrons are alike as far as we can tell and so are all protons. That
means that all the stuff, or matter, of our world is made up of two
kinds of building blocks, and all the blocks of each kind are just
alike. Of course you mustn't think of these blocks as like bricks, for
we don't know their shapes.

Then there is another reason why you must not think of them as bricks
and that is because when you build a house out of bricks each brick must
rest on another. Between an electron and any other electron or between
two protons or between an electron and a proton there is usually a
relatively enormous distance. There is enough space so that lots of
other electrons or protons could be fitted in between if only they were
willing to get that close together.

Sometimes they do get very close together. I can tell you how if you
will imagine four small boys playing tag. Suppose Tom and Dick don't
like to play with each other and run away from each other if they can.
Now suppose that Bill and Sam won't play with each other if they can
help it but that either of them will play with Tom or Dick whenever
there is a chance. Now suppose Tom and Bill see each other; they start
running toward each other to get up some sort of a game. But Sam sees
Tom at the same time, so he starts running to join him even though Bill
is going to be there too. Meanwhile Dick sees Bill and Sam running along
and since they are his natural playmates he follows them. In a minute
they are all together, and playing a great game; although some of the
boys don't like to play together.

Whenever there is a group of protons and electrons playing together we
have what we call an "atom." There are about ninety different games
which electrons and protons can play, that is ninety different kinds of
atoms. These games differ in the number