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I am new to blender, while making a cup i was doing an inward extrusion but it is not working. I was extruding the upper face of cylinder inwards to make it hollow like a cup but instead it's extruding a solid on the other side too instead of a cut. Please HELP!

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  • $\begingroup$ This could be a visual bug, try to scale your extrusion to fit inside your cup. The process you have described should be working. $\endgroup$ – BK. Jun 4 at 16:25
  • $\begingroup$ Did that from lot of suggestions , isn't working $\endgroup$ – Prashant Jay Jun 5 at 18:20
  • $\begingroup$ I guess the Subsurface modifier tricked you, but the answer below seems to have worked. You should accept that answer if it solved your issue. $\endgroup$ – BK. Jun 7 at 17:29
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It's not a bug. You extruded straight down, through the sides of your cup.

Blender isn't going to stop you from doing this, as it's not a real solid cup, just mathematical points connected in a conceptual 3d space.

I'm on mobile, so an illustration I sketched out in my notes app will have to do. Imagine this is the side profile of your cup. The green arrow is how you wanted to extrude, but the red arrow is how you actually extruded.

Rough illustration of side profile of cup and extrusion paths

Luckily it's easy to fix. Your extrusion was not wrong - just incomplete. The other step you have to do is scale in the face that resulted from the extrusion. Then it won't be intersecting awkwardly in a way that results in a non-manifold mesh. You can tell faces are backfaces (think: inside-out) when they're shaded darkly like that in the viewport. It's the first sign that you may have done something you didn't mean to.

Here's an illustration of what the side profile will look like after you scale it in to fix it:

Side profile of the edge in blue getting scaled toward the cup's center

You will want most 3D meshes you make to form a volume. There are a few exceptions (like leaf texture planes), but usually you want the mesh to be closed and not self-intersecting. Imagine: If it couldn't exist as a balloon (air-tight), then it doesn't have a volume.

Let's explore this concept with more sloppy drawings, shall we? Shaded all in green is the side profile of a cup that has a volume. Below that is the side profile of a cup as yours currently is, with self-intersecting faces that cannot form a volume. Marked in red are regions where backfaces will show as a result of the self-intersection.

Side profile of a mesh that forms a volume and one that doesn't

Is it starting to make more sense now?

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  • $\begingroup$ this was really helpful. thanks a lot $\endgroup$ – Prashant Jay Jun 5 at 18:07
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Your cup's extusion down is doing what Blender thinks you want, but not what you think you want. Extrusion (the 'E' key) travels at a tangential angle away from the average normal created by the vertices in your selection. Thus, they will continue in this straight path as you stretch it out (via mouse or input of a number on the keyboard). The issue you're seeing is that it won't automatically assume you want your mesh to stay within the bounds of the "cup". Here's a quick solution:

Make your cup: Cup with top selected

For these other steps, make sure your selection method is set to "Median" instead of something else: Median only!

Extrude the top like before, but hit escape rather than drag it around or inputting any numbers: Escape stops extruding with a value of 0.

After hitting escape, hit "S" (scale) and scale it down, to give it a little depth: S = scale

Now go ahead and extrude straight down like you did before. Your cup will look funky again, and the wrong side of the normals will show outside of the cup for the portion that overlaps: Notice the darker color on some faces?  Those faces' normals are facing inwards.

Now, with that circular face still selected, hit "s" again to scale it DOWN, making your cup more cup-like. No more weird normals sticking out!

This last image is just a top view of the cup, so you can see the interior circular mesh face: Such circle.  Much round.

Hope that helps!

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  • $\begingroup$ it did help. i am really thankful $\endgroup$ – Prashant Jay Jun 5 at 18:07

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