Scientific phenomena are occurrences in the natural and human-made world that can be observed and which cause one to wonder and ask questions.
Students should be able to dig deep into the phenomenon to uncover some ideas that are core to science. Phenomena are commonly thought to be the "ewwwww, weird, oh man!!" kinds of traditional science demos which (while still really fun for teachers & students) are not always academically productive. Phenomena should drive units and keep students working to "figure out" rather than "learning about."
–From Phenomena for NGSS