The thread about masturbation in the late 70 s reminded me of this classic.
A guy is in a cubicle in a public toilet when he hears a sound of someone knocking on the wall between his cubicle and the next one.
After a while , with the knocking continuing he thinks someone may be in trouble, so he looks over into the next cubicle. There is an old man sitting on the loo , trousers down and banging his hand against the wall.
''What's up mate?'' he asks.
The old guy replies ''First time I've got a stiffy in years and my hand's gone to sleep !''