What is an event? is like a message from one thread to another
How can events be signaled?
In Scratch, blocks whose labels begin with "broadcast" signal events whereas blocks whose labels begin with "when" handle events, the latter of which, recall, effectively represent threads themselves.
Describe IN YOUR OWN WORDS how sprites use events to communicate.
Two sprites might want to play Marco Polo with each other, with one sprite's behavior defined by the leftmost thread below and the other sprite's behavior defined by the rightmost thread.