The pin holds a thing called the "spoon" in place. It's secured with a hole in either side of it as it attaches to the "neck" of the grenade where the pin going through it secures the spoon to the neck of the grenade.
When the pin is removed, the spoon is free to come away from the grenade. Usually this takes more than gravity, but not "too" much more. Sometimes the force of removing the pin is enough to rattle the spoon loose/off from the grenade neck, triggering the fuse, in the case of most frag grenades like the one pictured above, the fuse length is pre-cut to be around 4 seconds, until it reaches the payload of TNT at the core of the fragmentation casing, exploding.
Depends on the grenade. Lotta people who talk about “grenade no work like that” are talking about a specific type of grenade they are familiar with.
Some grenades have very easy pins to pull, some grenades start the fuse as soon as the the pin is pulled, some require the spoon be depressed for the pin to be pulled.
Reality is, there are many types of grenades that work in different ways.
Table lamps have a long cord you can cut off and attach it's contacts to a metallic door handle after plugging it into an outlet. Imagine someone grabbing it and you jump from around the corner with your finger-pistols doing ZAP-ZAP sounds at them as their hand locks in a deathgrip over a knob and their body does Bethesda ragdoll physics. And it's still not a prank.
The prank is: I put it up because this harmful and incorrect information is more likely to hurt the prankster than their victim.