Can I syphon my friend's gas?
Why are you planning to steal you friends gas?
I have planned to syphon my friend's gas in order to get revenge.
I understand that you are upset with your friend. However, you should not steal your friend's gas as you could be leaving him/her in a bind when they are ready to leave. You should consider your need for revenge and you want to get back at your friend.
What are you talking about? I was planning to refill his/her gas tank later.
If you're just going to fill it back up later, then what is the point? People would consider this to be very petty, and not really constructive in any way.