In Scratch you are able to send messages within sprites to let them know what to do, for example someone says something then they send a message then the other sprite receives and replies to it. Here is how to do it;
Drag over the green flag and change sprite and so on…
Make the sprite say “Hi!” with the ‘say hello for 2 secs’ block,
Bring over the ‘broadcast message1’ block,
In the other sprite bring over the ‘when I receive message1’ block,
Make the other sprite say “Hello!” for 2 secs,
Click the green flag and see what you have done!
The messages tool in Scratch is handy for waiting in games and animations.