At the risk of dating myself, I remember pay toilets in the United States. You would go to a public restroom and there would be a contraption on the door of each stall in which you would insert a dime and turn the knob in order to be able to open the door. If you didn't have a dime, you were out of luck. Unless of course you were small, in which case you could crawl under the door, use the facilities, and then exit the normal way. The other option was to wait until someone else came out of a stall and hope that they would be willing to let you enter the stall without having to pay (even though they had to pay to get in).
It's been many years since any of us have had to pay to pee in the US. That is not the case here in Europe. In fact, finding any public bathroom facility in Europe can be challenging regardless of whether you are willing to pay or not. It is always best to first follow your mother's advice and to pee before you leave the house - this buys you at least several hours. Using the facilities in any restaurant you eat in (only available to patrons) or museum you visit, is also a wise choice. But again, these still may not be free even though you have just paid for a meal or paid a large admission price to go to the museum.
Most pay toilets in Europe involve a person sitting at the entrance collecting money. The advantage of this is that exact change is not required. The disadvantage, of course, is that you can't pile you and your two kids into one stall for the price of one. You therefore have to pay 2E (.50E each) for your family of four to pee. That's $3.00 US for the privilege of eliminating the 12E ($18 US) worth of soft drinks that you had for lunch (yes, that is only one soft drink per person). Occasionally there is just a change bowl with a sign posted of how much it is to use the facilities. Then you have the moral dilemma of whether to follow local custom and pay, to pay, but less than full price, or to skip paying and deal with your conscience later.
The ultimate pay toilet we encountered was in the parking lot just outside the city of San Gimignano. It was a large structure, but only one toilet for men/women/handicapped. You inserted your .50E and the door slid open like one of the doors on the Space Ship Enterprise on Star Trek. Everything else was automatic as well. You used the toilet and then pushed a button for 10 sheets of toilet paper to come through a slot. You walked over to a trough looking area and stuck your hands in. Soap was automatically dispensed. Move down the line and water was dispensed. Moving down the line further, warm air was blown in order to dry your hands. In the end you pushed a button and the door slid open again. You had a total of 15 minutes to do your business. We were never quite clear what would happen if you exceeded your 15 minutes. Would the door slid open and expose you to the world if you were not yet through? Once you were out, you heard a flush of water - I think some type of system washed the whole bathroom before the next person. The whole think was rather bizarre and a little space age. The good news is that a family of four could easily fit in as long as no one was too modest. In fact, two families of four could easily have fit in the large space with a net cost of .06 1/4 E (9 cents US) per person. What a deal!