He couldn't help but chuckle when he saw the puppy chasing its own tail.

chuckle 是动词，指"轻声笑、咯咯笑"，通常表示因为觉得有趣或可爱而发出的柔和笑声。这个词特指那种压低声音的、愉悦的笑声，比laugh（大笑）要含蓄温和。