Last night, millions of viewers watched and Twitter exploded as seventh-generation tightrope walker Nik Wallenda walked on a two inch-thick steel cord over the Grand Canyon. Wallenda, who is a devout Christian, could be heard praying throughout the 22-minute stunt, which he did without a safety harness or nets. In case you missed it, the special will re-air tonight on the Discovery Channel, and Mashable has posted a couple of YouTube video replays. Just how tense was the death-defying stunt? The expression on Joel Osteen’s face (who waited with Nik’s family on the other side of the canyon) says it all …