One promising approach to turn sunlight and water into fuel is to use mixtures of tiny nanoparticles, whereby different particles play different roles. For example, gold nanoparticles absorb sunlight well, but they can’t efficiently make fuel by themselves. They need particles of another material nearby. The trick is to transfer electrons produced by the gold donor particles, as they absorb light, to the acceptor particles that start the chemical reaction to produce. Now, scientists have found a way to count how many electrons transfer between the two materials, which could help develop more efficient donor-acceptor combinations.