When two shadows get close to each other, they seem to take on a life of their own. Suddenly they "blister," or bend towards each other and merge. Here's why.
Shadows include a penumbra. That's the half-shaded portion of a shadow where the object casting the shadow blocks some of the incoming light, but not all. Sometimes it can be dramatic. Other times, like in the video above, you hardly see it until the shadows jump together. This is called the shadow blister effect. Get two shadows close together and they'll take on a life of their own, stretching towards each other like they want to touch.
You can play around with this effect on a sunny day. Try reaching for a friend and watching your shadows hold hands before you do. Or look down at the shadows of leaves on a tree, and see how they stretch and distort to get closer to each other. There are few things that may make the illusion better. As the shadow-making object gets farther away from the surface onto which the shadow is projected, the penumbra gets wider, so the effect gets more dramatic. But because we live on a planet with an atmosphere, and that atmosphere scatters light, the farther away the object is from the surface, the less dramatically dark the penumbra is.
Play around with this on your lunch break, if you have the sun for it.
[Via Ray Theory Analysis of the Shadow Blister Effect, Bulging Shadows.]