As Hat blocks are designed to start a script, they are shaped so no blocks can go on top of them - this is done by rounding the top of the block. They have a bump on the bottom - this is so blocks can be placed under them. Hat block are called Event Hats as well, because they are events. Like clicking the green flag is an event.
Without Hat blocks, scripts can only manually activate. Each Hat block has a different method for getting activated - this is so different scripts can be started at different times. There are four different ways to activate a script - when the flag is clicked, a key is pressed, the sprite is clicked, or when a broadcast is received.
An example for when the When I Receive () block can be used follows: