The terms “broadcast” and “when I receive” are a little techie. I explain broadcast as a big shout, loud enough to wake up something that is sleeping. BBC often features in this discussion. When I receive means that the sprite has heard the broadcast. Sometimes it leads to a discussion about whether or not all the sprites hear the message but that only the ones programmed to respond will do so.
I’ve tried
a few projects but knock knock jokes are really successful. I have a pot
of jokes ready for children who don’t have a favourite one, you can click here
to access the file of suitable jokes. These
jokes work well because there is a clear and well defined sequence of actions,
ready to be controlled using broadcast and when I receive. The sprite that
starts the joke is the one in control, initiating actions using broadcast. The other sprite responds when a message is
received and then control reverts back to the first. By working with this
simple design, children understand that broadcast is a way of changing the order
in which blocks are executed.

The power point file I use for the lesson can be found by clicking here.