Okay kiddo, so do you remember learning about the Khmer Rouge in Cambodia? No? Okay, well the Khmer Rouge was a group of bad people who took over Cambodia back in the 1970s. They did some very terrible things, like making people work really hard for very little food and water, and even killing lots of people just because they didn't like them.
Now, some people today still don't believe that the Khmer Rouge did these terrible things. They are called "genocide deniers" because they deny that there was ever a genocide (which means killing lots of people because of their race or beliefs) in Cambodia.
But we know that the Khmer Rouge did do these terrible things because lots of people saw it happen and even survived the killings. We also have documents and evidence that prove it.
So it's important to always remember what happened in Cambodia and to not deny the truth of what the Khmer Rouge did. We want to make sure that bad things like that never happen again.