Like last year we used toilet rolls as a metaphor for the hollow cylinder that is the long bone's central section [diaphysis for you Latin-Nazis]. This year, instead of using A4 lab-books [average wt about 500g], I dropped by the office to pick up a box of xerox paper because A0 is exactly 1 sq.m in size [in the ratio 1:Sqrt(2)] and xerox paper is 80gsm, we know that a N=500 ream of paper weighs exactly 2.5kg and a boxful = 12.5 kg. Matt Parker raves on about A4 and why it's waaaay better than US Letter size <huzzah Europe>.
Four toilet rolls weighs very close to 25g, so if they can support a ream of paper that's a weight/load ratio of 1:100. We had two 16 y.o transition year TY kids auditing the class and I asked them to estimate/guess how much they thought their 4 "legs" would support - "form a hypothesis" in science-speak. By the time they had hesitantly suggested "1 ream", my most active group had stacked 5 reams, 12 lab-books, three boxes of latex gloves and an iPhone on theirs. That was close to 20kg or a load ratio of nearly 1:1000!! I think we were all surprised; and the rolls didn't look anything like failure.
Then one of the girls piped up saying that she weighed 59kg [call that 60kg!?] and maybe we should see how many rolls would be needed to carry her - and a wooden dissection tray to spread the load. We were conservative to begin and put 8 rolls down as support. A few iterations showed that 5 rolls [30g] was enough but 4 rolls [25g] too little <ploof!> to support a 60kg weight. The feeble looking low quality card-board cylinder is good for holding 2000x its own weight. Then a tall well-built chap cried "Me too!" and declared his weight as 113kg close to double his XX colleague's. A lot of toilet rolls were spread out under the wooden tray but they didn't stand a chance <frrrRUB> and it was man down and mangled cardboard. We then had a bit of a discussion about stacking corn-flake boxes at an aisle-end display in the local supermarket. Much hilarity, only partly tempered when stern Chemistry teacher came into to check on the TY kids. But I brought everyone back to earth with some further reflections on their femurs and how much punishment they are designed to take without breaking: it's a lot.
Verdict: a good way to while away an afternoon.