The cat sleeps all day.
Meaning: This sentence is saying that the cat is in a state of rest with the eyes closed for the whole of the time from sunrise to sunset.
Choice of Words: Sleeps means to be in a state of rest with the eyes closed. All means the whole of something. Day is the time from sunrise to sunset. The is used to refer to a specific cat, and cat is the animal we are talking about.
Alternative Expressions
Related Expressions