To Roll
To Tumble, To Lie Down, To Be Scattered About
intransitive verb, godan verb
You know that the kanji here means revolve. The verb version of this is similar. It means to roll or to tumble. When something is doing these things it is revolving, right?
ころがる
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Imagine something rolling down a hill. Inside is a very important uranium core (ころ) that you need to keep spinning. That's why they have it rolling down hill after hill, because they don't have anything else to keep it spinning like that, preventing it from exploding. This core is very important!
If you have trouble remembering the hiragana ending, just imagine a girl (がる) rolling down the hill too.
あんたのけいたいなら、その辺に転がってたわよ。
Your cell phone was left somewhere around here.
ダリンは、トーフグオフィスの前にフグの死体が転がっているのを発見しました。
Darin discovered the dead body of a blowfish, lying in front of the Tofugu office.
転がる石？イギリスのロックバンド、ザ・ローリング・ストーンズのことですか？
A rolling stone? Do you mean the English rock band The Rolling Stones?