How we know
Different tissues replace themselves on wildly different clocks. The epidermis, the outer layer of your skin, is among the fastest. Keratinocytes are born in the basal layer, push upward, and shed at the surface on the order of two to four weeks. Iizuka 1994 in the Journal of Dermatological Science measured this epidermal turnover time. The practical upshot: the skin you can see right now is only weeks old.
Blood is next. A red blood cell lives about 120 days, roughly four months, according to the standard StatPearls reference on the NCBI Bookshelf. Your marrow makes and retires red cells continuously, so the circulating supply is renewed over a span of months rather than all at once.
Bone is slow but never still. Through remodeling, basic multicellular units resorb old bone while others build new bone, replacing roughly 10 percent of cortical bone each year. The US Surgeon General's 2004 report on bone health describes how the adult skeleton renews almost completely over about a decade.
Then there is the famous big number. The claim that about 98 percent of the atoms in your body are swapped out every year traces to radioisotope tracer studies by Paul Aebersold of Oak Ridge, reported in the Smithsonian Institution Annual Report and summarised by TIME in 1953. Treat it as a memorable popular-science figure rather than a precise constant: real turnover varies by tissue and element, with body water exchanging in days and some structural atoms lingering far longer.
What it means
If almost none of your matter is the same year to year, what makes you still you? The answer physicists and biologists keep reaching for is that you are a pattern, not a substance. Richard Feynman put it plainly in 1955: the atoms come into your brain, dance a dance, then leave, always new atoms doing the same dance. Your individuality, he wrote, is "only a pattern or dance."
The cleanest analogies are a flame and a wave. A candle flame holds its shape while the gas burning inside it is constantly replaced. A wave keeps rolling forward while the water it passes through stays roughly where it was. Both are stable processes, not fixed objects. So are you. The matter is borrowed and returned; the arrangement is what persists.
The honest caveat
Be careful with the 98 percent line. It is widely repeated and broadly directional, but the original estimate is decades old and lumps together tissues that turn over at very different rates. Neurons in your cerebral cortex, the lens of your eye, and some tooth enamel are effectively with you for life. So the precise number matters less than the shape of the truth: your body is a river of matter, and you are the standing wave that holds while it flows through.