system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do "active cultures" get into yogurt?
gpt
To make yogurt, milk has to be inoculated with bacteria. Typically Streptococcus thermophilus, Lactobacillus bulgaricus, Lactobacillus acidophilus, and Lactobacillus casei. 

With out those bacteria, the milk would never be converted into yogurt. Active cultures mean that the factory did not kill the bacteria in the yogurt, so they're still alive when you eat the yogurt.