We love the sponge analogy. "Children learn languages so much faster than us, their brain is a sponge!" We trot it out at every family dinner, a bit like a collective excuse not to put in more effort.
The problem with the sponge is that it gives the impression that bilingualism will happen all by itself, in one summer, effortlessly, almost by contamination.
Spoiler: it won't.
We already accept that many things take time
No one expects a 2-year-old to eat neatly at the table on the first try. We know they'll make a mess, refuse vegetables three nights out of four, and that we'll have to repeat "eat with your fork" about 4,000 times before it sticks.
No one expects a child to learn to read in a week, to ride a bike without training wheels on the first go, or to be perfectly potty-trained overnight.