I did it a few years ago when I worked at a large block of offices in London. I went to the toilets, both men and women, and put a few drops of washing up liquid, you know ... the one you use for washing plates, dishes and cutlery in the kitchen ... I put a few generous drops in the cistern in each toilet. Just a few drops of super concentrated washing up liquid.
It was great fun when people flushed the toilets and the bowl filled up with bubbles everywhere overflowing onto the floor.
I stayed in my office giggling silently as news came from all over the building that people were getting millions of bubbles each time they flushed the toilets.
Some people flush the toilets whilst still sitting on them. They got the shock of their lives when their backsides were covered in soap bubbles.
I even put washing liquid in the main tank feeding the urinals in the men's room. As you know, the urinals flush automatically every few minutes to keep them clean and fresh. Apparently, they flushed whilst some people were standing there and the bubbles foamed all over their trousers. I heard one man seeing the bubbles said that he thought he'd had too much Champagne to drink the night before and it was all coming out now.
All in all, great fun was had by me ... and no one else. No one ever suspected who did it. The engineers had a great time trying to discover what was wrong with the system.
What fun you too can have this Halloween or when you next have guests. It is high time we all had a great sense of humour, don't you think?