1. O'Malley tries his hand at name-calling. When addressing the topic of compromising with Republicans on immigration, O'Malley didn't hold back, bashing Republican rival Donald Trump.

>> Click here to watch the clip

2. Sanders slams chatter about Clinton's emails ... again. At last month's first Democratic debate, Sanders made headlines and thrilled social-media users by saying of the Clinton email scandal, "The American people are sick and tired of hearing about your damn emails."

>> Watch the video here (start at 2:34 timestamp)

3. Sanders ties terrorism to climate change. The senator doubled down on earlier statements that climate change is the greatest threat to national security – and went even further by saying the two threats are connected.

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4. Clinton: Sanders is trying to "impugn my integrity." The debate heated up when Sanders slammed Clinton for accepting donations from Wall Street. 

>> Watch the video here

5. Sanders gets feisty about gun control. O'Malley touted Maryland's gun-control measures, to the chagrin of the Vermont senator.

>> Click here to see the exchange (start at the 11:51 timestamp)