From Test-Scratch-Wiki

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A screamer is a project where an image suddenly appears along with a random sound (traditionally a scream — hence the name "screamer"). These projects are made to scare people or to make a person laugh. Screamers can be fun and enjoyable, but they should not be too distressful, as Scratch is an all-ages website. If one sees a screamer project that is inappropriate or too scary, one should report it.[1]

The concept of a "screamer" is not unique to Scratch; the first notable ones were made starting from the year 2002. Today they are a common phenomenon on YouTube and other video hosting websites.

Creating a Screamer

Warning Warning: Keep in mind that this is a site for all ages. Never use frightening sounds or images. You can play a funny sound or a comical scream, with a cartoony drawing, but never use images that would appear in real screamers. It's also wise to add a scare warning in the project notes.

To create a Screamer, you need to first make a normal animation. Then add a new sprite, a 480x360 filled rectangle with your "scream" image. Finally, add this code:

when gf clicked
when I receive [scream v]
play sound [scream v]// optional
go to front

When your animation is over, you need to broadcast "scream" for your scream image to appear, along with the scream sound.