[techhelpbb]

Preface: depending on the day,
I am either the gentleman in the suit or the guy in the jeans.
Also I've had this happen a few times.

If they ask:
"I mentor students building robots as an educational tool to introduce them to engineering and science. This is not so much 'Robot Wars'. It's a cooperative challenge both on the field and off. It's a lot of fun and it helps insure that in the future we have more talent to help us all succeed."

If they don't ask and I don't know them:
"Hello. Wow...am I under dressed for this occasion. Sorry I gotta go mentor some students and my formal clothes are not suitable for building robots." If they don't care then oh well.

If they don't ask and I know them (there are thousands of people in my company):
"Hey, how are you? (Hopefully you get a response here.) I am off duty to go build some robots with my students."

I find that if people don't want to talk, they won't.
If people are interested they'll engage you if you let them.

For me I am also a big fella at 6'3" with a goatee.
So if I stand there postured like a security guard people will assume I am not all that friendly if they do not know me.
I suspect for other people that first unspoken impression would be different.
In reality that first <5 seconds can radically alter the entire situation.
I have a background in security and it's just human nature.