system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do shadows tend to be attracted by other shadows ?

I was in the bus today and whenever my shadow got close to another one, it would stretch to reach the other. I never paid much attention to it before now. I know it probably is something about light, but I'd like to know the exact reason of this phenomenon.
gpt
Imagine two equally tall mounds of sand on a flat surface.

Now, push the two mounds towards each other. You will notice that where the two mounds touch, the sand will combine and rise higher.

The same occurs for shadows. A shadow's edge is never completely sharp due to diffraction, a property of light that blurs the edges. The farther away the shadow is from the object blocking the light, the more blurred the edges become. 

When two shadows come close together, in the overlapping regions, the light gets blocked twice, once by each shadow. The amount of light blocked must be considered separately per shadow. Imagine that the blurred part each blocks 50% of light. Then one shadow allows 50% of the light through and the second shadow blocks 50% of that. Therefore, the total blocking is 75% which is why it appears darker than each individual shadow, much like how the sand in the two mounds rises higher than the original mounds as the mounds come closer and closer.