Disagree on stopping him outright, honestly. Controlled curiosity with safe items teaches way more than just saying no. Just gotta watch close and use something that won't wreck the machine, like a stale dinner roll.
Oh man, this reminds me of my cousin trying to "test" his new blender with a whole apple, stem and all. He swore it would be fine, said it was basically a smoothie. It sounded like a rock fight in there and the motor just died. He learned his lesson, but it was a smelly, expensive one.