What electronic part allows electricity to flow one way only?

Diode

How many players are on each side in water polo during active play? What European river forms part of the border between Germany and Poland? Which British explorer famously rescued explorer David Livingstone in Africa in 1871? Who created the android known as the Human Torch in Marvel’s Golden Age comics? Which group of laws enforced racial segregation and discrimination in the southern U.S. after Reconstruction?