Ask HN: Your biggest non-technical hack/discovery

Not sure if this qualifies as a 'hack', but when abroad and stopped by the police I suddenly only speak dutch. That seems to reduce the interest level considerably. Saved me a few fines by cops playing the 'fleece the tourist' game.

-In college, I found that the card readers to open the gates into the faculty parking lots, and into the inner ring of the campus, didn't really check for anything... other than a physical card in the slot. So 3 years worth of free parking close to campus was definitely great.

-I worked at a franchised sub shop for a time, and the music discs we were required to play were pretty horrible. Listening to Fallout Boy got really annoying after the 400th play or so. I decided to pull down the player, which could 'only' play the cds sent to us, to see if there was anything I can tweak. I found a series of 16 pins on the back, and with some googling, discovered the position to place the connectors so that I could play anything. It worked, and in no time, I turned the place into a hip hop club, making sandwiches to the tunes of Jay-Z and the like.

-Slamming the coin slots on games at arcades, dislodging a few coins.

-After taking a hot shower, I'll turn the knob so the water gets super cold for a few seconds. It's a nice way to get me going after a nice relaxing hot shower.

http://img.skitch.com/20091012-mt5p22p1fxi3s1y32wqid49pwm.pn...

Zachary Burt, University of Chicago, 2007

Assume n people are in a room and they each shake hands with every other person. This constitutes n(n-1)/2 handshakes: each person shakes hands with every other person. The number is divided by two to account for double-counting the handshakes.

The sum of handshakes can alternatively be computed by summing the number of unique handshakes made by each person: person 1 shakes n-1 hands, person 2 shakes n-2 hands (for he has already shaken hands with the first person), and so forth, until person n-1 shakes only one hand: person n has already shaken hands with everyone else. This can be expressed as: n-1 + n-2 + n-3 + ... + 1, or, through the commutative property of addition, 1 + 2 + .. + n-2 + n-1. This is conveniently expressed as the summation (i=1,i->n-1,i).

We know that the summation (i=1,i->n-1,i) is equivalent to the expression n(n-1)/2. If we replace n-1 with n, this is the same as saying (i=1,i->n,i) = n(n+1)/2. QED.