XKCD.COM has some nice strips.
Here Is a small guide in Disqus-formatting, that I have ‘published’ several times on the old GSMArena.com blog (with tiny variation).
UPDATE: Should in 2019 finally be more or less obsolete for wide enough windows!
Lightning up briquettes in a wood burning stove the top-down way, does not have to be complicated!
For some reason, surprisingly few know this extremely simple formula, please share it.
I have actually seen in a carpet store an employee unrolling the carpet to measure the length!!
[The formula is simple to prove too… r=kφ, L=∫r dφ=∫kφ dφ=½k(φ2²-φ1²)=½k(φ2+φ1)×(φ2-φ1)=½(r2+r1) ×n2π=(r2+r1) × nπ , q.e.d.]