This is What's Trending Today.

Americans often identify the two major political parties by the colors red and blue. The practice began with the 2000 election. Media organizations liked the idea because it made easy use of voting maps. It helped show the winner of each state as the results came in.

The Democratic Party is blue. The Republican Party is red.

For the last 16 years, there has been a lot of talk about red states and blue states. Since elections take place in autumn, red and blue have become "fall colors."

But this time of year, people on social media sites like Facebook and Twitter are also talking about true fall colors – like those found in nature.

The colors are orange, yellow, red, and brown. Why?

Huge numbers of trees change color in the late fall in the U.S. Their leaves go from green to shades of orange, red, brown and yellow.

A chemical called chlorophyll is responsible. Chlorophyll helps trees make sugar from sunlight. This feeds the tree and keeps the leaves green.

But, as sunlight decreases in the fall, the trees produce less chlorophyll. This is when the leaves show what some people call their "true colors."

The website SmokyMountains.com has a Fall Foliage Map. It predicts when people will see the best fall colors.

The best time to see the colorful leaves, also known as foliage, was in mid-October, but some areas in the U.S. still have colorful trees.

Twitter user, Chris Duncan shared a photo he took earlier this week in Washington, D.C. It shows a bright red tree and the Lincoln Memorial and Washington Monument.

One person shared a photo taken high above the Virginia Tech campus, a university in southwest Virginia.

But you do not have to be in the U.S. to see the color change.

Ewan Gunn posted a photo from Scotland showing the Craigellachie Bridge with beautiful green, orange and yellow leaves in the background.

And that's What's Trending Today.

I'm Dan Friedell.

Dan Friedell wrote this story for Learning English. Caty Weaver was the editor.