Some morning in the future, you take a pill — maybe something for depression or cholesterol. You take it every morning.
Buried inside the pill is a sand-sized grain, one millimeter square and a third of a millimeter thick, made from copper, magnesium, and silicon. When the pill reaches your stomach, your stomach acids form a circuit with the copper and magnesium, powering up a microchip. Soon, the entire contraption will dissolve, but in the five minutes before that happens, the chip taps out a steady rhythm of electrical pulses, barely audible over the body's background hum.
The signal travels as far as a patch stuck to your skin near the navel, which verifies the signal, then transmits it wirelessly to your smartphone, which passes it along to your doctor. There's now a verifiable record that the pill reached your stomach.