Hand/eye color coordination, finger dexterity, and non-sequential stacking capability, can all be measured with the stacking Hamburger or Sandwich, along with all of our "size color" related Building Block Sets. There is no right or wrong way to stack any of them. All of THE PUZZLE-MAN TOYS are non-toxic with a stain-like finish impregnated into the wood. Washable in the sink and tub safe. Also ideal for use as trivets and decorative hot-plates and coasters.