An 18" by 24" fresnel lens makes one hell of a fire starter.
Especially when focused on dead leaves that have had gasoline dribbled on them while you were cleaning off some tools. From a couple of feet away it makes a great death ray and keeps you out of the fireball that results.
( The Resident Loon Not a demographic. Do not measure., Wed 27 Jun 2012, 20:30, 2 replies, latest was 13 years ago)

could i, for instance get a maybe 18 foot by 24 foot version
and standing on a high roof top on a sunny day, focus the resulting eyeball searing white hot point of condensed sunshine on to a gaggle of pensioners on the path below, get the same result as i would if i used a smaller lens and some ants instead ?
( fluffybunnykiller Is feasting on the clitoris of life, Mon 2 Jul 2012, 22:14, Reply)

I'd think so.
Only in England focusing that much cloud would only make the poor dears think the sun was out.
( The Resident Loon Not a demographic. Do not measure., Thu 5 Jul 2012, 23:44, Reply)