Simple as that.

An American friend occasionally stayed at my flat when he was visiting London. Since I did not have a washing machine or dryer at that time, laundry had to be done at the local coin-operated launderette. He had to do the same, and headed off with his washing in a black refuse sack, the only bag big enough to take it.

He washed and dried his clothes, and bundled them in the black bag, and slung it over his shoulder. When he returned to the flat he was pale and trembling. I asked what had happened. He explained that on the way back, a policed squad car had screeched to a halt beside him, and two menacing officers had leapt out to confront him.

I should explain that this was at a time of IRA terrorist bombings in central London, and people, including the police, were on edge and extra vigilant.

 "Excuse me, Sir. Would you mind telling me your name?" demanded one of them. My friend did so.

"And where are you going?" Somewhat shaken by the inquisitorial tone, my friend hastily told him.

 "Do you mind telling me what is in the black sack?" he was asked. He told them it was washing.

"Do you mind if we look inside it?" It was asked politely, but refusal was not an option. He opened the bag to let them look inside.

"Washing," confirmed the policeman. "Simple as that, eh? Thank you, Sir. On your way."

The police were doing their duty to protect us, and doing it quite properly and politely, but it left my friend quivering from the experience. Since then the phrase "Simple as that, eh?" has been a catchphrase for me and my friends whenever a straightforward explanation makes something obvious and unremarkable.