How did Valve become involved with Sierra?

We were going around talking to publishers about the ideas we had for Half-Life. It turns out that most of them thought we were on crack, although they were polite enough at the time. I had a friend who knew Ken Williams, the founder of Sierra, and he kept telling me that we should go pitch Sierra. I was skeptical that Sierra would be interested, but it turned out that Ken had just barely failed to get the publishing rights to Doom and was really interested in making up for that mistake. He had been around enough Microsoft people to realize that we weren't as insane as we came across to other game publishers, and he pretty much signed up Half-Life on the spot.