Sit Up x Sit Down!

What is the difference between Sit Up and Sit Down?

That's what this post is about.

You can only sit up when you first lay down. It means to rise from laying down to a sitting position, to sit with the spine erect.
For example: Mike had already laid down, but he sat up to drink his tea.

Sit up also means to stay up later than the customary bedtime and to become suddenly alert.
For example: Everyone sat up right after the teacher mentioned the test.

On the other hand, you can only sit down once you are up. So, sit down means basically take a seat.

For example: Fortunately everyone could sit down on the bus.

Got it?