Hands! The freaking hands! My inability to draw hands has caused me to abandon 50% of my drawings. It's one of the many reasons why I draw Tiere not people. The picture below is one of my drawings.
When I draw humans its hands, feet, thumbs, noses, mouths and bodies.
Animals: it depends on the animal. If its something like a horse its the head and the hooves. Occasionally the body. When I draw things like Hunde I mostly mess up on the teeth and hind legs. Here's a drawing I did recently on my chalk board of a dog. Du can see how I messed up on the teeth.
Figuring out what to draw.... I end up just letting my hands do all the drawing. Barely anything I draw has any thought in them *shrugs*. But otherwise, coloring. I hate coloring. I'm a lineart type of person.
i fucking Liebe drawing eyes, but i'd have to go with hands and legs,\ (casually edits answer and adds one of his drawings because i can finally draw the other eye)
The hardest part for me is anything other than some form of Sonic character. I'm great at all Sonic characters, original oder fan-made, but I'm horrible at pretty much everything else. I can do vehicles really good too, though. Here's a pic of my Sonic FC, Xenos;
paws and sometime front legs. mainly because all I mainly ever draw are animals. but heres a pic of len I drew, just because I don't have any of foxes I drew yet!
