It's like pooping.

Speaking as a Chemistry Minor, synthetic items like guns and that shield she used in the Sports Festival are made of polymers. Polymers are a specific chemical compound, repeated and repeated and repeated. If you knew that specific chemical compound that item is made from…..Now I can see her failing to make something as a child, but as a highschool booksmarts genius, it's more believable.

In fact, metals are simpler than wood in structure, as wood used to be from a living tree.