snip

The practice of thinning emulsions is not a good idea IMO, despite what
the manufacturers say, because the added water isn't truly incorporated.
Think diluting custard :-)

/snip

Emulsion paint is not a true solution (e.g. like salt dissolved in
water). It is a colloidal suspension. Even so, it can be diluted with
no problems if it is stirred thoroughly during the addition of the water.

Same with custard: add water in small amounts and incorporate fully by
stirring between each addition. But don't paint the ceiling with
this... it is sure to come off :-))