Nine-year-old Oyon Ganguli of Waltham has a lot of ideas to make the world a better place.

One of them, which he calls The Cleaner, was selected as the winner of the second annual Mighty Minds contest, a national competition for youth inventors sponsored by Camp Invention and the National Inventors Hall of Fame.

Ganguli, an alumnus of the Watertown site of Camp Invention, won an all-expense-paid, two-day trip to the National Inventors Hall of Fame and Museum in Alexandria, Va., where he participated in the recent ceremony honoring 16 inductees and saw his prototype featured in an exhibit.

Ganguli, a rising fourth grader at James FitzGerald Elementary School in Waltham, said he got the idea for The Cleaner when his mother, Chandreyee Lahiri, participated in the ALS Ice Bucket Challenge two years ago.

“My mom’s friend in California complained about how much water was being wasted when there was a drought, and then we read an article saying a 10-minute shower uses 50 gallons of water,’’ Ganguli recalled. “That’s when I started thinking how a shower that filters and recycles water could solve the problem.’’

Ganguli and his classmate, Mateo Rosado, presented a filtration experiment based on The Cleaner in their third grade science fair, but excitement gave way to disappointment when research revealed that recycling shower technology is already patented. Ganguli then experienced those emotions in reverse, winning Mighty Minds despite assuming that his prototype and video demo would meet with more failure.

Ganguli said he was “blown away’’ when renowned astronomer Roger Angel mentioned him in his induction speech and several more inventors asked about The Cleaner. The encouragement has given him confidence to continue dreaming up inventions, such as a system to melt space junk into reusable metal and a self-watering mechanism for indoor plants.

“Inventing is fun, but it’s also hard work,’’ said Ganguli, whose career ambitions also include working at NASA, the LEGO Group, or Mojang (which developed the video game Minecraft). “Being around all these incredible people who changed the world taught me that it’s OK to fail, but not to stop trying.’’

Cindy Cantrell

Cindy Cantrell can be reached at cindycantrell20@gmail.com.