20. The fork in the road
This puzzle actually has a couple of solutions:
1. Ask either of the twins, "If I asked your brother the way to Eldorado, which road would he tell me to take?"
If you're talking to the truthful brother, he’ll direct you to the road that the lying brother would indeed recommend – which would be the WRONG road.
But if you're talking to the lying brother, he’ll also point to the wrong road; he’d lie and point you to the road that the truthful brother would NOT recommend.
Either way, you’ll know to take the other path.
2. Ask either twin, "If I asked you if the right-hand fork goes to Eldorado, would you say yes?"
If he's the truthful brother, he’ll simply say yes if the right-hand fork does go to Eldorado, and no if it doesn‘t.
The strange thing is, the liar will give the same answers.
If you asked the liar directly, “Does the right-hand path go to Eldorado?”, and it DOES go there, he’d have to lie and say “no”; so when you ask if he’d say yes to this question, he has to lie again and say “yes”.
If the right-hand road DOESN’T go to Eldorado, the liar would untruthfully answer “yes” to the direct question (“Does the right-hand path go to Eldorado?); so when you ask if he’d say yes to this question, he has to lie again and say “no”.
This riddle is very old, and has appeared in different forms through the ages. It’s based on a kind of logical problem called the “liar’s paradox”, first attributed to the 4th-Century BC Greek philosopher Eubulides. (A related problem is the “Epimenides paradox”, named after a 6th-century BC Cretan poet and philosopher who said in a poem that "All Cretans are liars.") The poet Philetas (4th-century BC), the story goes, puzzled so much over the Liar paradox that he wasted away and died of insomnia! His epitaph read:
Philetas of Cos am I
’Twas The Liar who made me die,
And the bad nights caused thereby.
Medieval scholars also studied these kinds of logical problems, which they called insolubilia, and they were important in 20th-century mathematical philosophy. “Liar’s paradox”-type puzzlers were popularised by mathematician (and stage magician) Raymond Smullyan in his 1978 book, “What is the name of this book?”
21. The F-words
Most people see three Fs – but actually there are six:
FINISHED FILES ARE THE
RESULT OF YEARS OF SCIENTIFIC
STUDY COMBINED WITH THE
EXPERIENCE OF YEARS.
Most people will not see the F in the word “of”. One reason for this is that the brain is subconsciously “sounding out” the letters and is scanning for the soft F sound, as in “fox” – so it overlooks the harder “v” sound in “of”.
Another explanation is that, when you’re reading fast, you tend to skip over small “grammar” words like “of”, “in”, “a” and “the” – you can leave these out and still understand the basic sense of a passage. In fact, this is a well-known speed-reading technique.