A pendulum swinging back and forth every second due to the law of gravity is a common sight. By going down to nanometer dimensions new phenomena emerge under different physical laws. According to a recent report, a potassium ion is found to swing back and forth inside a nanoscale tube at a terahertz frequency (a trillion times a second). Unlike the pendulum, the ion's oscillation is driven by electrostatic interactions with electrons inside the nanotube wall as shown in the figure. The tube, a carbon nanotube, is composed of a cylindrical hexagonal lattice of carbon atoms; the ion induces through a so-called dielectric response charges in the nanotube wall that interact back with the ion. This dielectric response of the nanotube electrons, ordinarily, can be described only through time-consuming calculations, but based on previous work (see Jan 2005 highlight) the response can now be calculated very quickly, in effect, on-the-fly along with the ion motion. The calculations revealed that carbon nanotubes attract ions into their inside and make them oscillate at Terahertz frequency. The Terahertz oscillator may serve as a detector in future imaging devices. (See also our nanotube website).