Math & Numbers Facts for Kids
Mind-bending number facts
A 3D object casts a 2D shadow, just as a 2D shape would cast a 1D shadow (a line). Mathematicians use this idea to reason about 4D shapes β their 3D 'shadows' can be constructed and studied even though we can't see them directly.
Two numbers are amicable if the divisors of each sum to the other. The smallest amicable pair is 220 and 284. The divisors of 220 sum to 284, and the divisors of 284 sum to 220. Pythagoras knew about this pair 2,500 years ago.
RenΓ© Descartes invented the coordinate plane (the x-y grid) in the 1600s, supposedly while lying in bed watching a fly move on the ceiling. His grid made it possible to describe geometry using algebra β a revolutionary idea.
You can only add fractions when they have the same denominator. To add 1/3 + 1/4, you convert both to twelfths β 4/12 + 3/12 = 7/12. The reason is that you must be adding the same 'sized' pieces.
The Barber Paradox asks: a barber shaves everyone who doesn't shave themselves β so who shaves the barber? If the barber shaves himself, he shouldn't; if he doesn't, he should. This paradox helped reshape the foundations of mathematical logic.
Tally marks are one of the oldest known counting systems, used by prehistoric humans over 40,000 years ago. The Ishango bone from Africa, dated to about 20,000 BCE, has tally marks that may represent prime numbers.
As a shape gets bigger, its volume grows faster than its surface area. This is why small animals lose heat quickly and large animals stay warmer β and why you cut food into smaller pieces to cook it faster.
Rajveer Meena of India recited pi to 70,000 decimal places from memory in 2015, wearing a blindfold. The record-breaking feat took nearly 10 hours to complete.
Whole numbers perfectly alternate between odd and even: 1, 2, 3, 4, 5... This pattern never breaks and extends to infinity in both directions on the number line. Adding two odd or two even numbers always gives an even result.
The multiplication table hides beautiful visual patterns. The nines column counts down (9, 18, 27...) while the tens digit counts up. If you shade all the multiples of any number on a 10Γ10 grid, you'll always see a diagonal or symmetric pattern.