system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are bulletproof vests not stabproof?
gpt
For a start, vests are not really bulletproof. They are bullet resistant. They work one of two ways. The lightest and most flexible ones use multiple layers of material, usually kevlar, that are strong enough to resist the bullet traveling through them. With each layer, the force gets spread over a wider and wider area, until it reaches the user and is (hopefully) spread out enough to not do lethal damage. A knife, though, just cuts the fibers; that being said, the vest will slow a blade down.

The other method uses high-strength armor plates, usually ceramic that absorb the impact but crack (and must be replaced); these plates are quite stab-resistant.