Why is -animateWithDuration:delay:options:animations:completion: blocking the UI?

Question

I always thought that Core Animation performs animations on the background. When I run this code, my UI interactions are blocked until the animation finishes:

Answer 1

Documentation is your friend :)

UIView Class Reference

Under animateWithDuration:animations:

"During an animation, user interactions are temporarily disabled for the views being animated. (Prior to iOS 5, user interactions are disabled for the entire application.)"

And of course, for all views under the animated view.