"Il pleut des cordes," say the French when it's raining hard.
"It's raining ropes," is the direct translation. It's the kind of idiom which can trip up an AI translator.
"You can imagine the dilemma this throws AI into," says François Yvon, a research director specialised in automatic translation with the French National Centre for Scientific Research CNRS.
He says a well-trained system will know that ropes are not literally falling from the sky and use the English phrase "it's raining cats and dogs".
"But is it going to find the equivalent when it's translating into Indonesian?" asks Yvon.
A comment on the weather is pretty low-risk, but more and more translation is being done by machines, and on topics with higher stakes than the weather.