system
Given the following post, respond as if you're commenting on Reddit:
human
How does "Plan B" work?
gpt
It is a large amount of progestin, a synthetic hormone that acts similarly to our own hormones, which acts to suppress ovulation (or the release of an egg from the ovaries). If there is no egg, then it cannot be fertilized and there will be no pregnancy. 

Sperm can live for up to 5 days, meaning that fertilization could happen at any point in those five days. If a woman ovulates, fertilization could then occur. It’s important to note that Plan B does not have much effect AFTER ovulation. So timing is important.

For example, if a woman ovulates the day after unprotected intercourse, and takes Plan B ok day 2 afterward, then it would not be effective and pregnancy might still occur. However, if she had taken it on day one, then ovulation would have been suppressed and fertilization would not occur.

In reality there may also be some effect of thickening cervical mucous and creating a tougher environment for sperm to meet the egg and implant, etc. But these effects are smaller compared to the primary purpose of suppressing the release of an egg, so that it cannot meet sperm and undergo fertilization.