Persistent Eye Tesuji
When you realize that one of your groups can achieve only one eye, you can use the Persistent Eye Tesuji. To perform this tesuji you need a drill. Once the eye is created you make a hole in the goban. The size of the hole should be sufficient to allow a stone to drop through. After that if your opponent tries to capture this group, he only loses his stone. Note that you may extend this tesuji to a more general case when you drill a hole beside an eyeless group which would create a Persistent Liberty, but beware of the fact that your opponent could take advantage of the liberty too to save a bigger group.
However you have to keep in mind that it only works with legged gobans. On "on-table" gobans, it only provides extra liberties for a semai instead of an extra eye. And if the opponent is alive all around he doesn't even need to waste turns filling the cavity - It's like any other group of dead stones - Removed at the end of the game! Reuven
Phelan: Even on a table goban, if you had an sufficiently large number of stones, they would pile up beneath the goban until they closed the hole from beneath. At that point it's as if it was a on-table goban. Imagine a group with 1E6 internal liberties! :p
Great. I already avoid playing chipped stones when they come out of the bowl. Now I have to avoid playing thin stones too, because it takes more of them to fill in holes in the board. JoelR
Wait, so if you play on top of your opponent's stone in the hole, would that capture it? Is this some new form of ko? Mef
I sense a tetris-go variant in the making... Truc
3D Go!!!! Peterius
HermanHiddema: To make this tesuji even more effective, place the lid of your bowl directly under the hole for free extra prisoners...
Anonymous: Or maybe we could put another go board that moves underneath so it could create a random game.