A better 10 rule is “No more than 10 words on a slide” (and in fact — zero is better). Brains don’t multitask, so never ask people to read and listen to you at the same time. It’s impossible. But we can listen while taking in visual images (movies have trained us for this). Visual images that complement what you’re saying increase audience engagement, as they connect those things and it makes them feel more informed and empowered.

As a professional speaker and TED alum, I have given talks ranging from 10 minutes to 3 1/2 hours with audiences equally (and fully) engaged, so the 20 minute rule only applies if you only have 20 minutes worth of content to share.

Here is the most important tip —give your audience value. Provide something they want and need to hear in a way that they are able and happy to hear it. Droning on about what you want to talk about, regardless of how long or how many slides, will lose them inside of 3 minutes.

At a conference last October, I was the morning keynote, so checked in the day before and watched the last four speakers of the day. All of them were saying some version of what I had been hired to talk about (many of them not well) and I watched the audience check out repeatedly. I went to the cocktail reception, had several conversations, and that night, restructured the talk entirely to be about the questions they felt were unanswered at the end of Day One. It was very well received. Very.

And it was an hour.
With 40 slides.
And a total of six words on all of them.