At the start of a critical political week, Theresa May finds herself under pressure for refusing to answer it.
Did she, or did she not know that something had gone wrong with our nuclear weapons, when she asked MPs to vote to renew the costly Trident system?
She wasn't in charge when the alleged misfire of the Trident missiles system took place - reportedly aiming off at Florida, rather than at its target. But as the prime minister now, when the mistake has come to light, she needs to look in charge of the facts.
Governments often use "security reasons" as a way, sometimes legitimately and sometimes more out of convenience, to avoid answering questions they don't want to. On Sunday, Mrs May didn't use that reasoning, instead repeating again and again an obviously prepared answer about the Commons vote to approve Trident shortly after she took over.
Plainly, she was not answering the very straightforward question from Andrew Marr. The obvious implication was that she did indeed know, but for political reasons, was simply not willing to admit it.
But overnight the government seems to have decided now to resort to that answer. Business Secretary Greg Clark has been using that "security" defence as a way of avoiding the issue.
But it's tricky because the government does indeed talk about weapons testing, even sending out press releases, and publicly awarding trophies to military teams when the tests go well.
When, as it appears they don't and the results are kept secret, the "security" excuse sounds less convincing.
So the simple "who knew" question will keep being asked. And for as long as the opposition parties keep pushing for clearer responses, ministers will keep looking like they are awkwardly, even shiftily trying to evade a straight question.
The irony is that it's unlikely that information about the misfire would have sunk the government in the vote to renew Trident in the summer. The majority was secure.
There would have been more debate about whether the weapons work, but it's unlikely the vote would actually have been lost. But the refusal now to answer questions over the mistake gives even more succour to the government's opponents.
And the refusal to say "who knew" allows those suspicious of ministers' motives to wonder what else we don't know.
It's one of the simplest questions in politics, and one of the most troublesome.