Twenty Eleven for mobile use (responsive?)

My website looks good on a desktop computer I think. But I want the site also usable for phone or tablet. I find lots of things on the forum, but none of them really matches my questions.

Those are:
1. does making usable on phone or tablet always mean making the site ‘responsive’?

2. and if so, is it possible to make a website on Twenty Eleven responsive?

3. and if not, is there another way to make my website usable for mobile devices?

Thanks in advance!