imprinting in ducks & geese - Konrad Lorenz 1935
Lorenz split a large clutch of greylag goose eggs into 2 groups.
One was allowed to hatch normally and the goslings followed their mother around.
Lorenz had the second group of eggs incubated and then arranged it so that he was the first thing the goslings saw  when  they hatched. From then on they followed him everywhere.
The goslings had formed a picture (imprint) of the object they were to follow.
When he marked the goslings as to which  group of eggs they had hatched from and then let them out together from an upturned box, each gosling went straight to its 'mother figure'. Lorenz' goslings showed no recognition of  their real mother.