Is there a reason why what follows the colon could not be simply "cooking"?
It doesn't sound very smooth.
It worked for me.
Why on earth does the machine not accept: "The chefs are working: they are cooking."???????
The problem may be the word 'chefs'. Strictly speaking, a chef is not just any cook but the head of the kitchen -- the 'boss cook'. Hungarian has its own word for 'chef' -- séf.
Because chef is a specialized type of cook; runs a kitchen and usually has specific qualifications