In a game I observed, one of the players failed to kill a group because he didn't know about this.
This is a very basic problem, but the principal is very important!
(;AB[rr]AB[rq]AB[rp]AB[ro]AW[rn]AW[qn]AW[qo]AW[pp]AW[pq]AW[qr]AB[qq]LB[qq:1]C[Black plays 1; how should white respond?]AP[goproblems]
(;W[qs];B[rs]
(;W[pr];B[so]C[Wrong; Black lives.])
(;W[or];B[so]C[Wrong; Black lives.])
(;W[so];B[pr]C[Oops...Black lives...]))
(;W[pr];B[rs]
(;W[so];B[sp];W[sr]C[Correct! White kills!RIGHT])
(;W[sr];B[so];W[sq]C[Correct! White kills!RIGHT])
(;W[sp];B[so]
(;W[sq];B[sr]C[Oops...Black lives...])
(;W[sr];B[sq]C[Oops...Black lives...]))
(;W[qs];B[so]C[Black lives]))
(;W[ps];B[rs]
(;W[so];B[sp];W[sr]C[Correct! White kills!RIGHT])
(;W[sr];B[so];W[sq]C[Correct! White kills!RIGHT]))
(;W[or];B[qs];W[pr];B[so]
(;W[rs];B[ss];W[sq]C[Correct! White kills!RIGHT])
(;W[sq];B[sr];W[rs]C[Correct! White kills!RIGHT])
(;W[sr];B[sq]C[Oops...Black lives...]))
(;W[rs];B[pr]C[Oops...Black lives...])
(;W[so];B[pr]C[Oops...Black lives...]))