Albert Barnes Commentary 2 Kings 20:10

"And Hezekiah answered, It is a light thing for the shadow to decline ten steps: nay, but let the shadow return backward ten steps." — 2 Kings 20:10 (ASV)

It is a light thing: It seemed to Hezekiah a comparatively easy thing for the shadow, which had already begun to lengthen, to simply make a sudden jump in the same direction. However, it was entirely contrary to all experience for it to change direction, advancing up the steps again after it had already begun to descend.