It might be pretty funny to make a poo joke, but it turns out there could be a pretty good reason why kids like to say words like 'poo', 'wee' and 'bum'.
New research from the University of Sussex suggests that, believe it or not, there could actually be a serious reason why kids especially love using these words.
Yep, an adult has spent time studying how kids at primary school use poop-related words!
Dr Perpetua Kirby, who did the research, says it's all about experimenting, and learning what it might be appropriate or inappropriate to say.
They can also be a really powerful way for young people to get across how they're feeling or to express their sense of humour.
She told Newsround: "Children enjoy playing with rude words like 'poo' because, just like adults, they want to play with different options that language offers them. Playing with words and ideas can be really impressive, hilariously funny, and also shocking and surprising.
"Using rude words can help to create shared funny moments with others, including adults, that is good for relationships.
"Children work out when they can use this language and are careful not to go too far. They also work out that it offers something above just making people laugh - it conveys lots of different feelings."
During her research, Dr Kirby was also told by one pupil that she stinks like a "hot bubbly poo bath" (!) but thankfully she found it quite amusing.
"I found it funny, but it also told me something about how they were feeling. It was a creative way to show that they were angry with me.
"For adults, listening to what children might be feeling when they use these words can be valuable, before shutting them down."