Pressing two keys simultaneously is pretty hard, I had to push them a few times to get it right.

I don't really understand what "one wood" or "one lead" is, so at this point there's no guessing. It would be nice to, for example, guess the density of a material, not the weight of one piece(?) of it.

Except for that, I really liked the art and sounds, they were really light-hearted and pleasant.

Without the rocks and wood etc being different sizes, it became more of a memory game than a guessing game. I like the idea of it though, reminds me of the "what's heavier, a kilogram of feathers or a kilogram of steel" meme. 

The A and E button inputs combined need a bit more of a window so you don't have to get them frame-perfect. Some simple background music would also be nice between the dings.

Decently made, but it took me a long time to figure out I had to press the A and E button at the exact same time. I pressed A first and then E and vice versa for a while. Some music would liven it up a little as well, but overall very nice.