Everyone who knew Flash said he was the smartest dog they'd ever met. He knew over 30 words and a dozen hand gestures that he mostly taught himself. He knew ball vs. frisbee, various people's names, left vs. right, "back up" as well as all the usual commands. He even started to recognize spelled words so that didn't work anymore.
Being such a smart and energetic border collie and with no sheep to work, he needed lots of games and activities to keep him busy. He loved to do his trademark tricks: roll over, spin, "be a bear" (sit up on his haunches and beg), and even "crawl" (which was more like scrambling with his front paws while doing a frog kick with his back ones).
A favorite game (for him, not us) was to look for a ball under the sofa, cabinet or chair and bark and paw frantically to get someone to get it out. You'd get down on the floor and reach for the ball while he watched intently. Then, as soon as he thought you weren't looking, he'd roll it under again with his nose and start the barking pawing routine til you got it out again. He'd do this all evening if he could find a willing sucker player.
Henry and Flash also played the Table Game, explained in more detail in this post. It was ritualized tag in slow motion. Flash would walk around and under the table with his frisbee or ball as Henry would try to catch it. Again, another one he'd do for hours.
He loved balloons, especially helium ones that had begun to lose their buoyancy. He'd bop them with his nose and play volleyball with us. We'd blow up regular ones for him to hold by their knot and carry around the house. Occasionally they'd pop but it never stopped him from wanting more.
He also loved to work at stuffed dog toys, open them up and methodically remove the stuffing. One Christmas when #1 Sister was visiting, she carefully re-stuffed one and sewed it up for him to take apart again while he waited patiently. They both amused themselves with this activity for some time.
Flash was known throughout the neighborhood as the spinny dog. Since he had an invisible fence, he'd run right up to the edge of the yard and stop to bark at passers-by. As he couldn't run into the street, he'd spin frantically in circles while barking at them. Unfortunately for him, instead of scaring people off, they usually laughed. It got so I'd describe where I lived by saying "I'm on the corner with that dog that spins" and people would immediately know which house.
So many wonderful memories of such a high-spirited and devoted dog. He had a complex personality, with clear likes and dislikes, full of doggy determination. We're not planning to get another dog in the near future. I can't imagine one without Flash's sensibility and communication skills. Just a regular dog wouldn't do.
And thank you all for the comments, hugs, notes on Facebook, remembrances and kind thoughts. Every pet owner knows this time will inevitably come, much as we try to deny it. And when it does, I hope you can each have the warm, peaceful, loving experience we had, heartbreaking as it was.