The old standby's seem to work.. Creatine, lots of food and cardio reduction..
I usually say, "Steroids of course!" and they say, "Get the **** outta here! Must be creatine!"
I'm never serious around people, so whatever I say they take as a joke..
It's easy for me, I tell them I'm refinining my diet to manipulate my body's insulin production via insulin sensitizing amino acids, and supplements like KR-ALA to coincide with times my body is more receptive to muscle growth coupled with an increased protein intake; specifically fast acting proteins during those times.
I also tell them I don't like to keep it up for too long because the rapid weight gain isn't great for blood pressure, and the eating habits in general wear me down.
They stop listening after the words "manipulate my insulin levels via...". No one suspects steroids as a result. They just think I'm some exercise genius or something.
I would go the diet route. Just say that you've finally got your diet straightened out and you're eating like a horse to gain that weight. If they continue to pry, just tell them that you're using some newer creatine and it works well. Tell them you're using Cr2 and that helped a lot. It's a win-win, they stop asking you questions and they spend their money on a product of a board sponsor. You can't go wrong.
As a chemical engineer, I'd just start babbling about biochemistry or in depth about HMB/creatine. I'd have people asleep in no time!