This simulation explores the design and physics of a wearable bioelectrical-impedance-analysis (BIA) sensor — the kind built into smart scales, fitness rings and wrist bands. A tiny AC current is passed through wrist tissue between two skin electrodes, and the sensor measures how the resulting impedance splits into a resistive part (R) and a capacitive reactance (Xc) that depend on the sensing frequency. In 3D, current is rendered as particles that either skirt around tissue cells at low frequency — because the cell membrane behaves as a capacitor that blocks slow-changing current — or cut straight through them at high frequency once the membrane's impedance collapses. Adjust true body fat and hydration status independently, then watch how a real single-frequency wearable's naive body-fat estimate drifts away from the truth whenever hydration is not "normal," exactly the limitation that makes BIA scales read differently before and after a workout.