I'm starting a new experiment tomorrow. I'm pretty sure it's going to work, at least for a while. I bought some Halloween candy. It will be put in a bowl on the table in my office. I will not say anything about it to the students, but they will figure it out as soon as one of them comes down to talk to me. Not a bad deal - a piece of candy in return for me asking them how chemistry is going. I would've totally gone for that when I was a grad student. There are lot of them who stop by a lot, or catch me in the lab or conference room, but some who don't. I am betting a few of them will stop by.
It's not exactly a secret that free food is a huge motivator for scientists. We know it, and we still get suckered in. It doesn't end when you graduate, either. Silicon Valley companies use the free lunch as a perk and a way to get people to work more. Cookies and coffee are the standard lure for seminars everywhere. In an ironic twist of fate, the grad students here are expected to provide coffee and refreshments when they do their oral exam and thesis defense, but perhaps that's more of an acknowledgement that profs like free food too, rather than a shakedown.