I'm going to stretch out my legs into somebody else's space on an airplane.
Why do you want to stretch your legs out?
I feel cramped.
This will just upset people. There is limited space on a plane and you taking up more space is rude.
I don't think anybody will mind.
Imagine if somebody stretched into your personal space. People aren't going to appreciate you doing that and regulations on airplanes are very strict these days. Try to resect other people's space when traveling on an airplane.