The main place to visit in Phnom Penh is the Khmer Rouge Killing Fields, which is actually a collection of sites where about 2 million out of the total population of about 8 million people were killed in the 1970s by the Khmer Rouge regime. In order to save ammunition, the Khmer Rouge executed this genocide by using all manner of horrific tools to try to kill as many people as quickly as possible. That included children, whose heads were bashed against the trunks of trees before they were tossed in mass graves next to their parents.