What you're saying is rather vague... can you give more information?
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If you mean a gravity system, here's how I do it:
[blocks] <when green flag clicked>
<forever if> << <not> <touching color[ black ] >>
<change y by( - 3 )
<end>
<when green flag clicked>
<forever if> <touching color[ black ]
<set y to( <y position> )
<end> [/blocks]
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