When you climb on a mountain, your goal most probably is the peak. Often you get very heavy legs doing it. Today my feet are an imaginary peak, which I try to conquer a few times with the help of my abdominals.
Lying on my back I distend my arms behind my head and lift my legs up ideally in a 90° angle. Then I bring my fingers to my toes and back down. I repeat this exercise as often I can until my abdominals feel like they crested the Mount Everest.
Lots of fun climbing!