Timing, non-compete, and during the time I work all IP belongs to them (Google).
I'm pretty sure you can negotiate this with HR at Google. I've seen it done for internships at Microsoft, so I don't see why Google would be any different. Most companies hire interns based on personal projects, so they're fairly receptive to letting interns continue working on them (in their own time, without Google resources).
don't work on your startup on-site + inform your supervisor & HR.