I got one of those too. Although it failed to say on the box:
“Warning, if you have son who is normally quite switched on, but doesn’t always pay close attention and doesn’t have a huge amount of mechanical aptitude, ensure that he knows the difference between the on/off switch and the fairly unique button which changes the length and functions perfectly well mid cut, then brief him correctly BEFORE he does the back of your head.”
I firmly, and personally, believe such a warning should be in place.