"Stop being childish!" This clear, simple instruction is used by teachers across the country. And yet, said to a four-year-old, it is clearly nonsensical.
The order to primary pupils was one of the comments which a teacher owned up to during a discussion of the silliest things said in class on The TES online staffroom.
Others have also fallen at this hurdle. One teacher told her charges to "grow up!"
Such attempts to defy the laws of nature are balanced by an apparent ignorance of simple natural processes. One contributor advised her pupils to stop wasting paper, telling them "it doesn't grow on trees".
Another told her pupils: "Don't come running to me if you fall and break both your legs."
For teachers determined to raise a smirk among pupils, though, the most effective route is through unwitting innuendo. For instance, one teacher owned up to a practical lesson in which she had instructed pupils to "put your balls in the sink, please".
Another asked her male pupils to help her tighten the connections on the swivel chairs in the computer suite. This activity demanded clear instructions: "Come on, lads. Hands on knobs, firmly now. And ... screw."
But contributors illustrate that it is often difficult for the harassed teacher to ensure that every sentence comes out exactly as intended. One recalls how, his mind distracted at the end of a lesson, he asked children to "all shit down". And a busy home economics teacher told her class that her recipe demanded the use of the "fooking coil".
Often, though, the most glaring clangers are the least expected. When one contributor upbraided an underachieving class with the words "do you want to spend the rest of your life stacking shelves in Sainsbury's?" she could not have anticipated that the response would be an aspirational "yes!"
And one new teacher quickly realised that there is one risible statement that will overshadow anything else a teacher may accidentally say: "Shall we do some work now?"