hitting your head against the rock, and in any case you will not feel a good helmet, it becomes part of you!
The helmet will especially protect you if you take a fall! (you or your colleague!!)
Make sure your helmet is adjusted correctly and be especially sure that it is tight enough for children. We often see kids with helmets swaying every which way!!! It's not mega-useful, you're not accumulating Likes on Facebook and especially in the case of a fall (or rockfall) the utility of the helmet is greatly reduced!
To stay put a helmet's strap must be clipped. The stories of guys who fell with their straps undone, those, you'll find in the cemetary.