Play-Doh, if you stretch it, remembers its shape for a few seconds. Water in a glass forgets the motion of a spoon immediately. And between them are fluids that combine the flow of water with the memory of Play-Doh: they are called viscoelastic. In such a fluid, micelles are dissolved, tiny clusters of molecules resembling a sponge made of spaghetti. When a bacterium-sized particle enters it, the micelle network wraps around it and remembers every movement.

Physicists placed a colloidal particle 2.7 micrometers in diameter into such a fluid and held it with a laser beam like tweezers. The laser created a well for the particle from which it could not escape on its own. But the particle kept jittering due to the thermal motion of molecules. The scientists recorded this jitter at a frequency of one hundred times per second, determining the particle's position to within five nanometers, which is ten thousand times thinner than a human hair.

To extract energy from the medium's memory, a double-measurement protocol was used. At the first moment in time, the particle's position was compared with a threshold: above or below. One second later, a second identical comparison was made. This yielded four possible states, each of which is linked differently to how the micelle network remembered the particle's past. It turned out that for one of the states, with a delay of about one second, the particle's energy first grows and then decays toward equilibrium. The micelle network, contracting around the particle, gives it the stored energy, like a spring that was first compressed and then released.

This effect made it possible to extract work that exceeds the energy stored in the particle itself. In an ordinary trap with a single well, there is no such excess because the particle relaxes too quickly. But if the trap is made with two wells separated by a barrier, the particle lingers and manages to take energy from the slow motions of the network. It turns out that the medium's memory works as a reservoir from which energy can be drawn if you know when to draw it.

Previously it was thought that the medium instantly forgets the past, and information about the system is irretrievably lost. This experiment has shown for the first time that hidden degrees of freedom of the medium not only exist but can be measured and converted into useful work. This approach opens the way to creating microscopic engines that use the memory of biological fluids or polymer solutions as fuel. Play-Doh, which remembers its shape, can now not only surprise but also work.