When I am taking group pictures I usually ask people to say “Kase,” which is German for “cheese.” I know it doesn’t produce the desired effect, except it does.
My understanding has always been that saying “cheese” moves your facial muscles into a smile – that’s why we say it when we are having our pictures taken. It makes us look more attractive (at least if you like smiles).
“Kase” doesn’t produce the same muscle movement, but I get the same effect. People laugh; they know I am telling them to smile.
Sometimes the words we use are less relevant than the sentiment behind them.