Most fragrances are made up of a mixture of smells, called notes. Each constituent smell/note has a differently sized molecule. The smaller molecules are the ones you smell first as they evaporate most easily, needing less heat energy to become gaseous. When you first put on a fragrance, you smell the top note/s most as these evaporate with little energy. As you wear a scent longer, more of the middle and base notes gain enough heat energy to evaporate and the smell changes. At the end of the day you may only be able to detect the base notes of the perfume as this is the only smell left on your skin, with a few of the bigger molecules not yet having enough energy to evaporate.
