Bullets is spraying, it's heating up a hundred degrees
Bullets are being fired. When bullets are fired from a gun the recoil and smoke causes massive heat, that can easily surpass a hundred degrees.
Also, not literally heating the air, but rather the whole situation in Vietnam. 100C is the boiling point of water. The war is heating up, becoming more violent and actually hotter and political situation in US is reaching a boiling point. Napalm was also used in Vietnam.
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