The coalescence of two water droplets happens so quickly, it’s essentially impossible to see, even with high-speed cameras. For this reason, researchers have turned to simulating molecular dynamics – essentially building computer programs that model the actions of all the molecules contained in the water droplets. Viewed this way, the very first contact between drops comes from thermal fluctuations – the random jumping of molecules across the separating gap. Once the bridge starts to form, it continues to grow, driven by thermal forces and opposed by surface tension. Eventually, this thermal regime gives way to the more familiar hydrodynamic one, where the bridge is large enough for flow to drive its growth. (Image credits: experiment – S. Nagel et al.; simulation – S. Perumanath et al.; research credit: S. Perumanath et al.; submitted by Rohit P.)