Hello, I was just wondering, I've seen quite a few projects where something (like spikes in a platformer) has its own sprite with costumes that are different variants of it, and when the project is run, more than one of the costumes are shown at once, like they are their own sprites.

Do you know how this magic works or do I have to ask griffpatch?

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This is done through clones. A simple setup for stuff like this is as follows

set [id v] to [0] // should be a local variable
repeat (10) // number of unique clones
change [id v] by (1)
create clone of [myself v]
end

This sets the clones up in such a way so you can individually control them.

when I start as a clone
if <(id) = (1)> then
do something
end
if <(id) = (2)> then
do something else
end