First off, let's leave all common sense aside. Common sense is going to derail the insanity that is this question =P I just went to buy some printer paper from Wal-Mart (not because I support Wal-Mart, but because it's a 30 second walk from where I work) and, foregoing my usual self-checkout as all of those lanes were preoccupied, I opted for the speedy 20 Items or Less checkout service. As I swiped my card and entered my pin number, the cashier was so nice as to double-bag my bulky printer paper, informing me that the corners are very sharp and will tear straight through one bag under the weight of the stack of paper. So I said thank you, got my $20 cash back and was on my way out. But I'm a sort of curious guy, so I started thinking... And here's my question to all you physicists (wanna-be physicists welcome too!). Now, I'm going to word this a little weird, but I think it will better illustrate my question. So let's say I'm in the market for a Katana (of COURSE my weapon of choice is the tonfas, but this is for illustrative purposes only) and I venture into my local Wal-Mart, because let's face it, their prices on Katanas are unbeatable. So I'm browsing through their fine sword collection and stumble across a beautiful one with a red handle, and it's really shiny to boot. It even matches my ceremonial kimono, and all at the low low price of $29.99! I ask the inately inept Wal-Mart employee if there are any more, and much to my chagrin I am informed it is the last one, and they have lost the box, but they are willing to part with their display model which I am repeatedly assured is just as sharp as any new one. So, being the impulse shopper that I am, I accept and head to the cash register. I hand the sword over to the cashier, she rings it up and as I swipe my card and start to put in my pin number she tells me she's going to be so courteous as to double-bag it, because the tip is very sharp and would puncture just a single bag easily. I'm not sure why she's elected to bag it like this; maybe that whole "hand scissors handle-out" thing. I would have been happy just to carry it, but whatever, I'll let her do her job. So now I have this sword double-bagged in Wal-Mart plastic bags, point down. My question is: Why is it less likely to puncture two bags than it would be to puncture just one? Wouldn't it just puncture the first one and then the second and fall through? Isn't there some infinitesimal amount of space between bag 1 and bag 2? Obviously double-bagging a gallon of milk is different; the weight distribution is comparitively vast, and it does not threaten to puncture the bag but instead to split the seam. So why would double-bagging sharp objects help? I mean obviously it does help, but why? And if we're going to get hung up on the elasticity of the plastic bags, why would it work with paper bags also, supposing I am eco-friendly?