Step 1: Paste Active Objects into backgroud

The two animated blocks are Active Objects and therefore can not be set to an obstacle. So, we must paste them into the backdrop at the start of frame.
This is a really good time to play some Mario music. The sample music.ogg is embedded into the mfa. Simply click Play and loop sample and the file will appear in the samples list. Note that playing a sample 0 times will continuously loop the sample.
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