You need to put one cup of water in a pan and simmer it until it has reduced by half.
(I can't think why.)

Student or Learner
You need to take one cup water in a pan and simmer it until it remains half of it.
Is my sentence correct?
You need to put one cup of water in a pan and simmer it until it has reduced by half.
(I can't think why.)
If you reduce it too much, you can always add more water.
Just be careful you don't dilute it too much.
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Stir gently and cover for 10 minutes. After cooking, leave to stand for a further two minutes. Serve in a glass.
Voila - half a glass of warm water!
I am not a teacher.