On the main poster of the Fight Club, Tyler Durden (Brad Pitt) is holding a pink bar of soap in his hand. If I remember correctly, Tyler was a soap maker.
But why soap? Is there any hidden symbolism behind it?
In addition to the answer @Yetisasquatch gave, chemicals used in the making of soap are also used to make the explosives that go off in the credit card companies at the end of the movie. The fact that they use human fat is also relevant because Tyler Durden's philosophy in the film revolves somewhat around the disdain for gluttony and people going through life as consumers vs. doing something meaningful or rewarding.
Well it's not hidden. Yep he made soap ..... out of human fat or blubber if remember a few funny scenes. There's a scene where they have a bag of human blubber! It adds probably a great creep factor to the character IMO.
While @gannolloy's answer is correct if you're rewinding the script to come up with a way to blow up the buildings, it doesn't answer "why soap?" except in retrospect. "Tyler Durden was the guerrilla terrorist of the food service industry." He also spliced porn into movies as a projector. He lived in a debilitated and abandoned warehouse. Given these attributes, it's fair to say that he wanted to stay off grid while sticking it to the man. Making soap killed two birds with one stone. It allowed them to make money and it also allowed them to be "selling rich women their own fat asses back to them."
Your fist won't collapse during impact if you're holding something reasonably rigid in it. Like a bar of soap in a prison. It's not as good as brass knuckles (which also protect the outside of your fist) but you'll feel a difference if hit by an empty fist and one holding an appropriately sized rigid object.