Why people shouldn't mess with me

Today was a beautiful day in Oxford, so I laid out in University Parks to read. Not long after I'd started, five teenage kids sat on a bench near me. Shortly thereafter I started noticing a bunch of pebbles landing around me. I ignored the kids for a while, assuming they'd get bored soon enough. After all, they had pretty pathetic aim. Eventually they did get bored, but just bored enough to up the stakes. They started creeping closer, wondering how close they could get before I'd finally notice them. Eventually when the most obnoxious of the little runts got a few feet away from me I informed him that I wasn't an idiot and would appreciate it if he and his little wanker friends would kindly leave me along. Naturally they then started playing the "what are you talking about I don't know anything about pebbles" card. After a little more verbal pestering they finally ran along.

About five minutes later they returned with friends. I'd already learned they were only 15 and they weren't hostile, so I was more annoyed than intimidated. From my original encounter it was clear that they were bored and just wanting someone to pester and talk to for entertainment, so I developed a good plan. As they approached I told them I'd talk only if they could solve a maths problem I gave them. I wrote down an infinite geometric series and asked them to find the sum

They were fairly bright and one of them quickly told me it was one. The others argued with him saying it wasn't really one, it was naught point nine repeating. I then proceeded to demonstrate the equivalence of the two. By this point in time the most obnoxious of the group had grown bored. He seemed a little obsessed with his recently discovered genitalia, so he preferred drawing pictures all over my paper while I regaled the others in some more maths. Several of them were vaguely familiar with the notion of differentiation, so we talked about basic calculus, the exponential function, the concept of Taylor series, etc. It worked out quite nicely since the interested guys kept the others in check. Eventually the group ran along, but I convinced three of them to stick around long enough to see the derivation for what is perhaps the most beautiful formula of elementary mathematics.