Answer to
old chestnut: Lisa's dog:
The diagram below shows the answer schematically.
A, B, C, D, and L represent the people (L = Lisa), and 1, 2, 3, 4, * represent their dogs, respectively. {} is the boat. Remember that A, B, C, D, L, and * can row.
ABCDL1234* {}
ABCDL34 -> {*12}
ABCDL34 {} *12
ABCDL34 {*} <- 12
ABCDL34* {} 12
ABCDL4 -> {*3} 12
ABCDL4 {} *123
ABCDL4 {*} <- 123
ABCDL4* {} 123
DL4* -> {ABC} 123
DL4* {} ABC123
DL4* {C3} <- AB12
CDL34* {} AB12
CD34 -> {L*} AB12
CD34 {} ABL12*
CD34 {B2} <- AL1*
BCD234 {} AL1*
234 -> {BCD} AL1*
234 {} ABCDL1*
234 {*} <- ABCDL1
234* {} ABCDL1
4 -> {*23} ABCDL1
4 {} ABCDL123*
4 {*} <- ABCDL123
*4 {} ABCDL123
-> {*4} ABCDL123
{} ABCDL1234*