1) The word "book" contains two vowels: O and O.

2) The Danube River, flowing through ten countries in Central and Eastern Europe, is the second-longest river in Europe. It has played a significant role in the cultural and economic development of the regions it traverses and is a popular waterway for river cruises.

3) The phenomenon known as "black holes" occurs when matter is crushed into an infinitely small space, creating a gravitational pull so strong that nothing can escape.

4) The human body contains enough carbon to make over 9,000 pencils.