You are taxing my memory it been a lot of years scince school but here goes ==matter must have three demisions occupy space and have density or weight for instance a peice of paper has all of these and a shadow only has two===a peice of paper no matter how thin it is its thickness is its 3rd demension ===a shadow has a hight a width but no density or weight nor does it occupy space it is simply a absence of light hope this helps ==i would suggest a little time in a library for more info on matter

