Starting with the structural aspect of the challenge, you're doing pretty well in most cases, although there are some areas where your constructions end up somewhat oversimplified, which can cause incorrect spatial information to be conveyed to the viewer, or for elements to be flattened out. For example, on 15 it looks like you've only drawn the outward-facing faces of your rims' spokes, but may not have included the side planes. In most other cases, you did include the side planes of these spokes, although in cases like 20 as shown here you overextended the far edge of that spoke's side plane beyond where it would have been cut off by the inner surface of the rim.

I also noticed some hesitation in the execution of your smaller lines when drawing your rims/spokes - this is understandable to a degree, as those smaller lines are considerably more challenging, but some cases like those on this page suggest that you may be forgetting to hold to the principles of markmaking and use of the ghosting method from Lesson 1 as strongly as you should.

Aside from that, you're generally doing a good job of building out your wheels with multiple ellipses, ensuring the middle one is larger to create an impression of an "inflated" structure - something that'll land with a bounce, rather than a heavy thud.

Continuing onto the textural aspect of the challenge, in this regard the challenge serves as something of a trap. It's quite common to see students reach this stage of the course, but being as far removed from the earlier lessons, they frequently show that they may not have gone back and reviewed that material, or may not have held as strongly to ensuring their warmups cover all the different topics/exercises introduced up until that point. This results in them tackling the textural elements of this challenge without actually applying any of the concepts introduced in Lesson 2 - like the use of implicit markmaking over explicit markmaking, and drawing the shadows their textural forms cast, rather than the forms themselves.

This is definitely something you appear to have fallen into, as we see a ton of explicit markmaking - that is, outlining of your textural forms or individual planes - as well as cases where you focus more on drawing what you see, as you see it, without considering the specific forms that what you see suggest to be present, and deciding what kinds of shadows to cast as a result.

There are definitely some cases where you've leaned more towards thinking about texture in a manner that is closer to correct. For example, 14 and 18 are cases where you appear to be working less with line, more with filled areas of solid black. This is a step in the right direction, although you are currently filling in the side planes of those textural forms, not working with cast shadows.

A good rule of thumb is that if you find yourself filling in a shape that already exists in your drawing, you're probably engaging more in form shading rather than cast shadows, as a cast shadow will generally require you to think about the relationship between the form that is casting the shadow and the surface that is receiving it, and using that to design the shadow shape. So, if you find yourself filling in an existing shape, take a step back and ask yourself why you haven't had to design a new shape.

When it comes to comparing the results of these two (drawing and filling separate shadow shapes versus filling in existing shapes), it can be a little difficult to tell the difference because those differences are subtle. Here's a diagram I drew to help another student understand the difference, along with the explanation I gave them:

In the top, we've got the structural outlines for the given form - of course, since we want to work implicitly, we cannot use outlines. In the second row, we've got two options for conveying that textural form through the use of filled black shapes. On the left, they fill in the side planes, placing those shapes on the surface of the form itself, and actually filling in areas that are already enclosed and defined on the form and leaving its "top" face empty. This would be incorrect, more similar to form shading and not a cast shadow. On the right, we have an actual cast shadow - they look similar, but the key point to pay attention to is shown in the third row - it is the actual silhouette of the form itself which is implied. We've removed all of the internal edges of the form, and so while it looks kind of like the top face, but if you look more closely, it has certain subtle elements that are much more nuanced - instead of just using purely horizontal and vertical edges, we have some diagonals that come from the edges of the textural form that exist in the "depth" dimension of space (so if your horizontals were X and your verticals were Y, those diagonals come from that which exists in the Z dimension).

The last thing I wanted to discuss is explained in this diagram. It explores a common mistake when dealing with textures relating to holes, grooves, cracks, etc. - really any situation where the "named" element corresponds not to an actual physical form, but rather to negative space, or an absence of form. In such cases students tend to want to simply draw them directly and fill them in, rather than considering the textural forms in question actually being the walls that surround these empty spaces, and cast shadows upon one another's surfaces.

With that, I'll still be marking this challenge as complete. As it is meant to be a trap, it's really more about informing students that they should probably go back and review anything they may have neglected or forgotten, and to consider what kinds of subject matter that might be. At the very least, I would urge you to review the following:

• The principles of mark making and the ghosting method from Lesson 1

• The texture concepts from Lesson 2 - you'll want to review it all, but these reminders are a good starting point