Liquid water is often seen as a problem in electronics because it can cause damage and corrosion. On the contrary, here we show that a concentrated aqueous solution can be used as an ultra-fast switch at terahertz frequencies. The relative variation of the transmitted terahertz field decreases by -20% at pump-probe overlap and recovers with a fast time constant of ~70 fs. As this liquid terahertz switch is much faster than the cycle duration of terahertz radiation (1 ps), the perturbed liquid can be seen as a temporal boundary that clips the terahertz field and varies its frequency content. This work demonstrates the frequency up-shifting of terahertz radiation in a water solution, from about 2 THz to 3 THz and beyond, with an efficiency of 4%. These results might have important consequences for pulse slicing, coherent control, and frequency combs at terahertz frequencies. This could open the door to the completely new field of "water electronics".