One of the first things you learn when you give presentations is that a canned demo is much, much safer than a live one. As in Murphy's law (Whatever can go wrong will go wrong, and at the worst possible time). Our five-year had to learn it all by himself though.
He was playing with stacking cups this weekend. "I can stack all the cups with the small cup at the bottom," he claimed.
"OK," I said, "show me."
He proceeded to place the smallest cup on a nearby table.
"Why don't you do it on the carpet?", I asked.
"If I put the cups on the carpet, then it will fall down," he replied and proceeded to explain the reason. "Because the carpet is bumpy." (His rationales, when wrong, are usually interesting)
Unable to quit while he was ahead, though, he continued, "Let me show you."
He moved the smallest cup from the table to the carpet and then placed the next larger cup on top of it. And wouldn't you guess? The tower didn't tilt. "Two cups won't fall," he recovered smartly, "three cups and it will fall."
I have been there before (haven't you?), so I was curious about how he was going to handle the situation.
He placed the third cup on top. The funnel stubbornly remained standing. "With four cups, it will fall," he said and put the fourth cup on top. Still no go.
"It will fall," he said emphatically, "that's why I'm going to do it on the table."
I'm going use to use that the next time a demo fizzles out -- "It will work," I'm going to say emphatically, and move on.