A 5-year-old girl has become the guinea pig in her own science experiment as she tracks the progress of a silver coin through her digestive system.

Niamh Aageson was playing with the coin in her mouth two weeks ago when she breathed in and accidentally swallowed the 50- or 20-cent piece.

An x-ray showed the coin had gone down her oesophagus and lodged at the entrance to her stomach. Since then, the family have been tracing the coin through her stomach and into her lower intestine, using a metal detector.

Doctors say the coin might emerge in the coming days - or it might get stuck somewhere and never emerge.

Now Niamh has a sage warning for other kids: if it isn't food, don't eat it.

Niamh spent the afternoon at Starship hospital until doctors could see the coin had passed into the intestine.

"We were lucky the milk worked and she didn't vomit because it could have got stuck on the way back up," Niamh's mum Astrid Aageson said.

"As soon as it gets through to the stomach it is not so dangerous."

Most parents wait for the coin to pass naturally but Niamh's uncle thought using a metal detector from his work at an engineering firm would be a better option.

"We have been scanning her stomach every few days and the coin is still there," Astrid said. "It has been almost two weeks now so we are hoping it will be out soon."

To read the complete article, see:She's keen to see cash waste away(www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10829548)