Soldiers might soon have lighter, slimmer, and more-flexible armor that lets them move more easily but still protects them from bullets and shrapnel. Dubbed liquid armor by its developers at BAE Systems, Arlington, Va., the new protective material incorporates shear thickening fluid (STF) and Kevlar. The liquid is a proprietary mix of nanoparticles, possibly silica or sand, suspended in a nonevaporating fluid like polyethylene glycol. STF, a non-Newtonian fluid, stiffens milliseconds after it is struck. And when it stiffens, it locks the strands of Kevlar yarn in place, making the Kevlar more resistant to penetration and letting it spread the force of impact over a larger area. In tests, 10 layers of Kevlar treated with STF provided more protection than 32 layers of Kevlar alone.