The frog boil metaphor illustrates how it's easy to miss small changes that build up over time until it's too late. The scenario, as told, is that of a poor frog who would leap away from hot water but, if put into cooler water that is gradually warmed, won't respond in time to getting boiled.
As a metaphor, it describes a powerful and pernicious shortcoming in how we perceive the world. A student kitchen that gets messy mug-by-mug, a business with gradually slowing sales, a river that is slowly polluted, a road that gets busier each day, a bay that sees fewer fish each year, content that drifts, cancer that slowly spreads, a story or behaviour that becomes normal, a website that gets slower feature-by-feature, or a climate that slowly warms are all cases where we may not be happy where we end up, but we didn't notice how we got there.
It's hard to respond to gradual change, especially when it spans generations. Jared Diamond called it out as a potential fate of the Easter Islanders and how they could have cut down the last tree. One barnacle is nothing, but many barnacles can cause significant headaches for big ships.
Fortunately, there's a flip side. Many, many things are getting better without us realising. Small positive changes, such as in attitudes, education, or healthcare, add up over time—the Destiny Instinct can mislead us. Fast and slow layers make a system, like a forest, more robust. Getting one percent better each day leads to a 37x improvement in a year. And enough molehills can make a mountain.
What a frog does, in this case, is not, apparently, true—don't try it—it's much better as a metaphor, not an experiment.
The frog is Ed Emberley style.