Bill Gates leads the search for a high-tech toilet
Millions of people across the developing world live without easy access to safe, clean toilets.
This week, the Bill and Melinda Gates foundation in Seattle held the "Reinvent the Toilet Fair", inviting scientists and engineers to showcase the latest in toilet technology.
"If you're a poor person living in a slum, there's either no toilet at all, or you'll have to go a long ways to a facility that may be unsafe to get there," said Mr Gates, the billionaire founder of Microsoft.
At the fair, inventors showed off toilets that turn human waste into fertiliser, electricity and fuel.
The BBC's Alastair Leithead was at the fair.