Okay kiddo, are you ready to learn about Lech, Czech, and Rus? They're three countries that used to be around a long time ago, before you and I were born!
So, a really long time ago, there were three brothers named Lech, Czech, and Rus. They all lived in a place called the "Great Moravian Empire," which was pretty big and important back then.
The Great Moravian Empire eventually fell apart, and the brothers each went their own way to create their own countries. Lech went to Poland and created what's called the "Lechitic" group of languages, which includes Polish. Czech went to the Czech Republic (which used to be part of Czechoslovakia) and created the "Czech-Slovak" group of languages, including Czech and Slovak. Rus went to Ukraine, Russia, and Belarus (which used to be one big country called the Soviet Union) and created the "East Slavic" group of languages, including Russian, Ukrainian, and Belarusian.
So, even though the Great Moravian Empire isn't around anymore, we still have these three countries and their languages because of Lech, Czech, and Rus. Cool, right?