12 is a number.
However "12" is not a number,
because it is in quotes and
that makes it a string.
If we try to add a number
to a string, then we will get an error.
If we want to convert a string
into an integer, then we need
to use the to_i method.
to_i is a short form for to integer.
Let's fix the last example
by using to_i.