A seat was deliberately left empty during Barack Obama's final big speech as president where he sets out what he still wants to achieve.
Usually, during the State of the Union address, the chair next to the first lady is filled by a high achiever or guest who reflects an issue important to the president.
But this year he left it empty to remember victims of gun violence.
The hashtag #emptyseat started to trend.
President Obama has recently been pushing for a policy change to toughen up gun laws in the US.
The White House revealed the seat would be vacant earlier this week.
It follows a run of mass shootings. Some of the tweets using the hashtag #emptyseat featured the victims from those.
Other tweets featured those killed at Sandy Hook Elementary School in Connecticut in 2012.
The hashtag, which is still being used, has already been tweeted more than 16,000 times.
Last year, US journalists Alison Parker and Adam Ward were shot dead by a gunman in Virginia, while broadcasting live on air.
Pro-gun groups responded pretty fast too.