What are your head and your feet? Fluids are flowing in and out of them, outer layers are sloughing off, they're warm enough to be quite bright in infrared, potassium-40 in your head and feet are beta-decaying and so on. A decent fraction of the momentary mass in your brain may be in your feet four heartbeats later (and vice-versa). Are you implicitly considering them in some time-averaged way?
(A comment similar to yours would be that your head is older than your feet because of general relativistic effects. Even if we ignore everything before you reached adulthood, there's a Ship of Theseus thing going on thanks to tissue turnover on timescales of weeks to months e.g. https://en.wikipedia.org/wiki/Bone_remodeling_period ).
(A comment similar to yours would be that your head is older than your feet because of general relativistic effects. Even if we ignore everything before you reached adulthood, there's a Ship of Theseus thing going on thanks to tissue turnover on timescales of weeks to months e.g. https://en.wikipedia.org/wiki/Bone_remodeling_period ).