system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a laxative work?
gpt
It depend on which type of laxative. There are osmotic, stimulant, lubricant and bulk agent laxatives.

Osmotic work by forcing water to stay inside the intestine. It also attract water from the surrounding tissues, therefore increasing the water content. Adding water increase the volume of the stool triggering a reflex. When nerves in the intestine sense that it's almost full, it trigger the need to go to the toilet.

Bulk agent have a similar mechanism of action but, it use dietary fibers. Since we can't digest these fibers, they pass through our digestive system unchanged. They also absorb water which increase their volume. Adding more volume trigger the same mechanism described earlier.

Stimulant laxatives are irritating to the intestinal walls. The intestine react by increasing its peristaltism as a way to get rid of the irritating agent as fast as possible. Peristaltism is the mechanism by which muscles along our digestive tract force food (and stool) down the tube until the undigested remnants are pushed through the anus.

Lubricant laxatives act by coating the intestinal walls with an oily substance. Since it's oily, it's also slippery making it easier for the stool to go down the tube.