Create audio that is only on one side?

Does anyone know how to create audio that appears only on the left or only on the right in stero? Even if I select 100% Left panning, the audio is still audible on the right side of my headphones and vice versa.

I am using Audacity 2.1.2 with Windows 10

That’s a problem with either your headphones or playback system. If a track is panned 100% to one side, then Audacity will play that track through one channel only.
Check your sound card settings and ensure that no effects are enabled. “Environmental” effects (such as “Concert Hall”) will create reverberation in both channels even when there is only sound in one channel.