Time for another English idiom! Today's idiom is "a dime a dozen," which means very common.
As you may know, a dozen means 12. We use this word frequently to talk about buying things:
a dozen eggs:🥚🥚🥚🥚🥚🥚🥚🥚🥚🥚🥚🥚
a dozen donuts: 🍩🍩🍩🍩🍩🍩🍩🍩🍩🍩🍩🍩
a dozen roses: 🌹🌹🌹🌹🌹🌹🌹🌹🌹🌹🌹🌹
And a dime is a coin worth $0.10 in America:
So, if something costs a dime for a dozen, it's really cheap, so it must be really common. This is a metaphor, though, so it isn't literal. Here are some examples:
Beach towels are a dime a dozen in Florida.
Many people think old baseball cards are valuable, but they're a dime a dozen.
Cell phones used to be for rich people only, but now they're a dime a dozen.