What are the simplest unsolved problems?

Answer 1

The obvious issue with unsolved problems is that we can't tell exactly how hard they are until we've solved them, but if a simple problem can mean that it is easy to understand what the problem asks about, I'd like to nominate Goldbach's conjecture .

The question just requires primary school arithmetic to grasp, it simply asks whether or not every even number greater than 2 is a sum of two prime numbers.

We have yet to discover every insight required for the answer .

Answer 2

We are in constant need of energy, electric or chemical.
At the other hand, we have a surplus of energy, which is heating up our planet.
But that energy is at such a low level that we can't convert it.
So the conversion of low level energy to electrical energy is a simple, but unsolved problem.