Use function in multiple controllers

[eluser]jj135[/eluser]
Hi! I/m new to Code-Igniter. Trying to build my fist application. It's not that hard to get started... Documentation is pretty good.

I have a question: How do I create a function I can use in more than one controller? For example a logout function. Or a function that checks if a user is logged in? I can create this function in a controller itself and use it ($this->functionname()) but if I want to use this function in multiple controllers, where do I store it and how do I call it?

Thanks!