What are solid-fuel missiles, and why is North Korea developing them?

North Korea launched a long-range ballistic missile into the sea off Japan's west coast on February 18 after warning of a strong response to upcoming military drills by South Korea and the United States. A day later, North Korea confirmed it was an ICBM, tested in a "surprising launch" drill aimed at confirming the weapons' reliability, its state media reported. Watch this video to find out why is North Korea developing solid-fuel missiles.