Goldbach’s Conjecture says that each even number is the sum of two primes.
For example, 6=1+5, 8=3+5, 18=11+7, etc.
Sounds good, doesn’t it?
The only problem is that up till now, nobody has managed to prove or disprove it. We do not know for sure that it is correct or false – though we do have good reasons to think it is true.