My mom taught me that when you are thanking someone for something you always say gracias por. That makes sense because someone exchanged something to you by giving you something and you are thanking them for exchanging something to you. Por is used for exchanges. Then I said Tengo un libro por ti and my mom corrected me saying para ti. That makes no sense because I have a book for someone which means I am going to exchange this book to them by giving it to them. Por is used for exchanges. Tengo un libro por ti. Gracias por el libro. I need an explanation for why it is Tengo un libro para ti instead of por because para is not used for exchanges. Para actually translates to in order to. Like this: Necesito un lapiz para hacer mi tarea. Please help.