Let's say that we made a sphere inside a sphere...

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And now, we want to rip open that outside sphere in order to see (part of) the sphere underneath.

What tool would we use?

Why...the RIP TOOL of course.

Go into Edit mode and select the vertices or edges where you want the rip to happen, press "v", and then use your mouse to indicate which direction to rip. Use the mouse to align/position the rip the way you want. Done.

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Peace

James