Effective lessons are punctuated by opportunities stimulate curiosity, puzzlement and hopefully, awe.
This is particularly effective at the beginning of lessons and teacher demonstrations, (especially of discrepant/counter-intuitive events), can be a powerful way to set the stage for deep and meaningful learning.
I must state, that I feel a physical demonstration in the classroom is always preferable but another quick and convenient method is to use brief video clips or pictures.
There are some interesting items there, my favourite being the explosive seed dispersal.
The site also gives you, the science educator to submit similar phenomena. So check out the collection and if you can, add to it!