5 Logical Riddles That Will Train Your Mind Time for some mental exercise. Kill your boredom and solve these mind-boggling riddles. If you think you’re a pro at them, this test will prove it within minutes. Find out where you stand.   There is an ancient invention still used today in some parts of the world which allows people to see through walls. What is it? There is a common 9-letter word in the English language, such that if you keep removing its letters one by one, the resulting 8 words are still valid. What is this word? Remark: The removed letters do not need to be from the beginning or the end of the word. Puzzil Primus was born on the 314th day of year 20B.C. and died on the 271st day of year 20A.D. How old was he when he died? What four-letter word can be written forward, …