acid base - How does the litmus pH indicator work?

I'm wondering about how does the litmus solution work as a pH indicator.

And another question:

When you put drops of litmus solution into an colorless acid (e.g. $\ce{HCl}$), it turns red. But then if you add colorless base (e.g. $\ce{NaOH}$) to the mixture, it turns blue. How come?

I thought that the litmus solution doesn't change color after it's changed color once already after a chemical change with an acid or a base. In this case, I thought that the mixture wouldn't turn blue after adding the $\ce{NaOH}$ into the mixture. But it seems that it still works well after used once.

How come?