In a feat of miniaturization that makes your Apple Watch look lame, scientists have implanted tiny lasers within living cells. The lasers can be used to track individual cells for days and weeks, the researchers report this week in Nano Letters. A laser requires two things: a material that can be stimulated in some way to produce light and a "resonant cavity" that will ring with light waves of particular wavelengths much as an organ pipe will ring with sound waves of distinct frequencies. Light resonating in the cavity stimulates the material to emit even more light, greatly amplifying the light to create a laser. Researchers had previously used living cells to fashion lasers by loading the cells with fluorescent proteins and placing them within a resonant cavity. Now, a team of physicists and biologists have gone a key step further, coaxing a cell to envelop a tiny plastic sphere that acts like a resonant cavity—shown in green in the micrograph above—thus placing a whole laser within a cell. The spheres are seasoned with a fluorescent dye, so that a zap with one color of light makes them radiate at another color. The light then resonates in the sphere, triggering laser action and amplifying itself. Crucially, each laser shines at a couple of distinctive wavelengths depending on the precise size of the sphere, as shown in the graph. So although demonstrated only in cultured cells, the technique might someday be used to track the movement of individual cells, say, within cancerous tumors.