Which set of instructions is most formal?
Fill up the bag with some water until it’s almost full but not totally.
Fill the bag with water, leaving one inch of space at the top.
You should fill the bag with water and leave some space at the top.
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The second instruction, “Fill the bag with water, leaving one inch of space at the top,” is the most formal. It uses clear and direct language, avoiding colloquialisms, and provides a specific measurement for clarity. This precision helps ensure that the instructions are followed accurately.

In contrast, the first and third options are more conversational. The use of phrases like “some water” and “you should fill” gives them a less formal tone. Being explicit about the quantity, as in the second instruction, is a hallmark of formal communication, making it ideal for instructional contexts.