Earlier this week, Democratic presidential candidate Martin O’Malley got booed when, speaking at the Netroots Nation conference, he responded to a group of #BlackLivesMatter activists by telling them that “all lives matter.” He was later forced to apologize.

O’Malley isn’t the first person to fail to understand why “all lives matter” is a tone-deaf rallying cry for a national politician in 2015. Hillary Clinton did the same thing earlier this year, though she has since corrected herself. And lots of white people have expressed confusion about why it’s controversial to broaden the #BlackLivesMatter movement to include people of all races.

The best explanation we’ve seen so far comes from Reddit, of all places. Earlier this week, in an “Explain Like I’m 5” thread, user GeekAesthete explained, clearly and succinctly, why changing #BlackLivesMatter to #AllLivesMatter is an act of erasure that makes lots of people cringe.

GeekAesthete explains:

Imagine that you’re sitting down to…