I tried this several times with different size pieces of paper and in different locations within the bowl. I found that the paper only ignited in one specific place in the bowl and that the longer and thinner the pieces were, the easier they were to burn.
The brown paper never ignited into a flame, but it clearly was burned by the sunlight.
You see, the bowl has a parabolic shape, which means that when the rays of the sun reflect within it, they meet at a single point. If you hold an object, like a piece of paper or a grapevine, at that point, the rays get concentrated enough to raise the temperature of the object and cause it to burn, Mike Tuts, professor and chair of the Columbia University physics department, told CNBC.
(Source: Mike Tuts, Columbia University)
In fact, it is the same method is used to light the Olympic torch before the ceremonial games.
The torch is placed in a parabolic mirror, which like the bowl, focuses the rays into a single point and ignites the fuel in the torch, creating the flame. The tradition dates to the original Olympic games in Greece.
The science explained by Tuts would occur in any metallic bowl, not just the one from IKEA.
“One small point is that you want the object you are setting on fire to be small, because if it were large then it would obscure the sun’s rays and there wouldn’t be any light to concentrate,” Tuts said.
That explains why the thinner pieces of paper were easier to burn.
On a larger scale, this phenomenon occurred in London in 2013. A skyscraper dubbed “the Walkie Talkie” was built in such a way that it would act as a concave mirror.