A few years ago, English was the only language which most primary school kids in England spoke.
But now more and more kids speak a different language at home, so for them, English is their second language.
Usually it's because they were born in another country, or their parents moved to the UK from somewhere else.
At most primary schools in England around one in every seven kids speaks a different language, but at some, that number is much higher.
At one school in Middlesbrough in the north east of England there are kids from 35 different countries.
That means that six out of every seven kids speaks a language other than English.
It can make lessons more interesting and help kids learn about other countries and cultures.
But some teachers say it can make their jobs harder.
The government says it's giving extra money to schools where lots of languages are spoken.
They say they don't want kids to be at a disadvantage at school because they don't speak English at home.