I do that with our guinea pigs.

They get a cookie* every day at 5:00 pm when I get finished with work. After 5:00, whether l come in the back door or just come downstairs from my office, they start wheaking and chewing on the bars of their pen (which they think encourages us to give them food for some reason).

Sometimes, after I’ve given them their cookie, I’ll go back up to my office. When I come back, they start acting like they didn’t get the cookie.

I just walk up to them and tell them that they already got it and they aren’t getting another. They stare at me for a moment, then just go back about their business.

I don’t think they actually forget they they got it. I think they just hope I might have forgotten. But it is interesting that all they need me to do is tell them they got it already and they give up.

* The cookie is a vitamin C supplement made of pressed hay with molasses as a binder. The molasses is why they lose their minds over them.