I just wanted to say it is kind of cool how the levels build off of each other. Once you figure out the OR and then the XOR, it really helps when designing your half adder. It's a weird combination of those two. In other words, an XOR would work great for a half adder if all you had to worry about was the sum bit (and not the carry bit).
Yeah! In some ways I think OR is the hardest level, because for me the design for AND was very natural, but for OR it just felt awkward and wrong.