Halve it if even, triple it and add one if odd. Does every number eventually reach 1?
Start with any positive whole number. If it's even, divide it by two. If it's odd, multiply it by three and add one. Repeat the process on whatever number you get. Try it with a few starting numbers, like 6, 11 or 27, and see where the sequence goes. Does every starting number eventually spiral down to 1, or could some number climb forever, or loop without ever reaching 1?
Reveal the answer
Nobody knows for certain — it's one of the most famous unsolved problems in mathematics. Every number ever tested, including all numbers up to past 2^68, eventually reaches 1, but no one has proven it must happen for every possible starting number. The problem was posed by German mathematician Lothar Collatz in 1937. Mathematician Paul Erdős reportedly said "mathematics is not yet ready for such problems," and it remains unsolved despite being simple enough to explain to a child.