A parent touring her child's soon-to-be kindergarten in the US this week was troubled by a poem taped to the chalkboard.

The nursery rhyme, written in large, colourful letters, appears to follow the same tune as the lullaby Twinkle, Twinkle, Little Star, but tells the children what to do in the event of a school shooting.

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Lockdown, Lockdown Lock the door Shut the lights off Say no more Go behind the desk and hide Wait until it's safe inside Lockdown, Lockdown It's all done Now it's time to have some fun!

"This should not be hanging in my soon-to-be-kindergartner's classroom," Georgy Cohen, from Massachusetts, said in her tweet.

Thousands of people liked and commented on Ms Cohen's post, with many expressing their dismay at seeing the song displayed at the school.

But in a reply Ms Cohen said:

"The school is doing exactly what they need to be doing, and I am glad for it."

"My issue is with the political & cultural factors that brought us to this sad state. Please talk to your legislators about the need for gun reform."

Why do kindergarteners need a song about school shootings?

Since 1999, 170 schools and about 150,000 students in the US have been affected by school-based gun violence.

There have been two mass schools shootings this year, at Parkland, Florida and Santa Fe, Texas.

The 2012 massacre at Sandy Hook Elementary School, in which 20 children between the ages of six and seven were killed, is the fourth deadliest mass shooting in the US to date.

Somerville mayor Joseph Curtatone and superintendent Mary Skipper, whose districts include the kindergarten with the Lockdown song, told the Huffington Post the poem was a result of "the world we live in".

"Students in Somerville and across the country know how unnatural this is," a statement from the pair said.

"Yet we all know that one of the most important roles we have as educators and community leaders is to ensure that all of our students and staff members are safe and prepared in case of an emergency.

"This poem is an example of how one of our educators used a rhyme to help her young students stay calm and remember the key steps they would need to follow during a drill or real emergency."

Should we teach kids about such dark things?

Using rhymes, cartoons and jingles to prepare children for dangerous situations is not new — and it can be useful.

At the height of the Cold War, schoolchildren in Chicago practised taking shelter under their desks at 10:30am every Tuesday morning.

Alarms and sirens would sound outside and the children would watch the short film Duck and Cover about how to survive a nuclear attack.

The film's star was Bert the Turtle, and the opening jingle went like this:

There was a turtle by the name of Bert And Bert the Turtle was very alert When danger threatened him he never got hurt He knew just what to do He'd duck and cover Duck and cover

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Earlier this year, a false nuclear missile alert in Hawaii made people think for almost 40 minutes that the world was going to end.

Many didn't know (or at least didn't follow) official advice about what to do in the event of a nuclear attack.

Some experts have argued it's time to bring Bert the Turtle back.