Generally speaking just about any 5th grader could tell you when a flower is open, but as some of my students learned, that isn't always so easy. Here's a member of the Annonaceae, the custard apple family, Guatteria diospyroides, that demonstrates my point. Don't most of these flowers look open? And the circled ones look closed, but these closed flowers looked just like the open ones just a couple of days ato. The flower buds get bigger and then the perianth parts open, 3 whorls of 3 tepals, of which you can readily see the inner two whorls. The open tepals expose a "button" that consists of a large number of flat, "leafy" stamens closely appressed to each other and tightly surrounding a cluster of pistils. This takes a couple of days and the flower sure does look open, but there is no fragrance, the pistils are not receptive, and no pollen is being shed. After another couple of days the inner whorl of tepals begins closing again until they are pressed up against each other leaving a small opening in the center (two circled flowers). Now this flower is "open" in the sense that it now emits an odor, a tropical fruity smell plus nail polish remover, and the pistils are glistening and receptive. At the end of that day the perianth opening completely closes, and no one enters or leaves for another day at which time the stamens finally shed their pollen and shatter falling into the perianth chamber. Flowering is now over and the inner two whorls of perianth are shed leaving only the sepaloid outer whorl and the pistils; this releases any pollinator held within. It takes a lot of marking and observation to figure this out, but it gets even more complicated when you notice that all the "open" flowers on the tree are in sync, either pollen accepting or pollen dispersing, tick-tock, every other day. Trees that are ticking get pollinated by trees that are tocking, back and forth, day after day throughout the flowering season, as beetles move from tree to tree seeking a food reward. If these trees are like some of their relatives in SE Asia then all the trees that are ticking or tocking will be the same year after year. Bet that will take my students a long time to figure out, but right now some of them like the idea of staying here.