Surprise?

Last night, I helped Ben make cinnamon rolls for work. At some point early on he looked at me and said, "I'm having trouble keeping a surprise from you." I said something like, "Okay," and kept on with the work.

He does this every once in a while. I'm never quite sure what to think. Should I dread or anticipate this unexpected thing? Or, should I just do as usual and ignore it and occasionally pry at him? I did query if it was time/date sensitive. It is. Apparently, if he tells me when I should expect the surprise, then I'll automatically know what it is. Haha, me? I can hardly get a joke, let alone figure out what strange things lurk in the depth of my husband's id.