The International Space Station (often called the ISS) is a research laboratory about the size of a football field that orbits Earth about 250 miles above our heads. Unlike any other building humans have ever constructed, the ISS floats in space, circling our planet every 90 minutes at over 17,000 miles per hour.

Sixteen countries worked together to build the ISS, including the United States, Russia, Japan, Canada, and nations across Europe. They launched the pieces into space one at a time starting in 1998, then astronauts assembled them like an enormous jigsaw puzzle while floating in space. The station has been continuously occupied since November 2000, meaning there have been people living aboard it every single day for over two decades.

Astronauts on the ISS conduct experiments that can't be done on Earth. In the station's microgravity environment (where everything floats because there's no up or down), scientists study how plants grow, how flames burn, how crystals form, and how the human body changes without gravity's constant pull. These experiments help us understand everything from developing new medicines to planning future missions to Mars.

The station orbits fast enough that astronauts see 16 sunrises and sunsets every day. If you look up at the right time, you can sometimes spot the ISS with your own eyes: it looks like a bright star moving steadily across the night sky.