but it only became widespread in the 1920s. This was after Morton introduced magnesium  

carbonate to salt, ensuring a more  free-flowing pour of the salt. 

Before this, salt was kept in cellars  and broken into clumps or kept in bowls  

with spoons. The introduction of magnesium  carbonate revolutionized salt storage and usage. 

That's why The Morton Salt company later adopted  the slogan "When it rains it pours", meaning  

that its salt containing magnesium carbonate  would not stick together in humid weather. 

. As ceramics became more common,  

the design and popularity of shakers evolved,  turning them into collectibles and souvenirs. 

The Smithsonian highlights that  most salt shakers have one hole,  

while pepper shakers have two or three. The  Great Depression boosted the popularity of these  

shakers due to their affordability. The pairing of salt and pepper,  

despite their distinct flavors and uses,  led to them being marketed together.