Just because you can, doesn't mean you should.
This is one of those seemingly simple life lessons most everybody learns in childhood and then spends much of their adult life forgetting. It's generally taught metaphorically in response to a child's attempted justification - "Johnny did it." - for doing something incredibly stupid, dangerous or both. "And if Johnny jumped off a bridge would you?" is the dominant variant of the clichéd answer. As a public service, I'd just like to say, "Depends on the height of the bridge and what's underneath it," is not the answer your parents are looking for. It's a rhetorical question, dude. The only appropriate answer, as it turns out, is remorseful silence.
The antithesis of this rule, and the avenue for real learning, is: just because you might get hurt, doesn't mean you shouldn't give 'er a go anyway. There are any number of valid, scientific reasons a six-year-old might jump - or, let's be honest, urge his friend to jump - off the roof of a house.
Testing whether an open umbrella might slow your fall sufficiently to keep you from getting badly hurt if you decide to try that technique on a higher structure, a bridge, say, is one. It doesn't, in case you were wondering.
Refining the experiment using a parachute of your own design cut out of a canvas tarp shows real scientific curiosity. Assuming you wait until your friend gets his cast off to test it also shows real human compassion as well. The fact neither of you have any grasp of aerodynamics is no reason not to do it; it is, in fact, the vast pool of ignorance you're trying to overcome in the first place by such an experiment. And sadly, no, it doesn't work either. But it does provide some insight as to why parachutes aren't made out of a single, round piece of canvas.
Just because you can cut your sister's bangs with your grandmother's pinking shears isn't a good enough reason. Just because you can grow Sea Monkeys TM in your mom's best crystal bowl isn't a good enough reason. Just because you can play your armpit like a symphonic farting instrument isn't a good enough reason... especially in church... during the dramatic pause in the minister's sermon... with your parents sitting less than an arm's length away.
Unfortunately, many of us, most of us even, forget that basic lesson as we grow older and smarter. Wisdom does not come with age. Knowing more stuff comes with age. Knowing even more devious ways to transfer the risk to your less bright friends - complete strangers even - comes with age.