Hello, this is my first post. I'm following a project in a book and mostly it explains everything. But in a push button circuit it doesn't say what the link to the SDA GPIO pin (physical pin 3) actually does. The book talks about what switches do, but the way it is wired doesn't look to me that the button is doing what switches normally do - I'm sure it is but I can't see it! I've googled the SDA pin and searched the forums but can't work out exactly what is going on. I assume info is being fed to the PI that the button is depressed or not - but how does that complete or break a circuit?