You have the same skin, same bones, the same meat. More importantly, when speaking about the mind, the essence of your good self, you have the same brain: the same neurons (more or less), the same synapses, the same blood vessels and connective tissue inside your skull.
More profoundly, you have the same memories. And unless something alarming happens to you like an almighty thwack upon your head, you have the same impression of an unbroken continuum of consciousness.
But the more you think about it, the more you realize you are not the same person you were even a second ago. You may be composed of the same atoms, but only approximately. In one minute, for example, you will metabolize about 4.5 grams of oxygen - that is one quadrillion or thereabouts every 60 seconds. Several trillions of carbon dioxide molecules leave your lungs and trillions more will be assimilated from the products of digestion.
In short, the cellular chemistry that makes you tick is a constant merry-go-round of breakdown and synthesis.
On a molecular and atomic level you are not quite the same person you were a minute ago.