Dr Jennifer Cleary shows RTÉ 2fm's Stephen Byrne a great way of showing the difference in density between oil and water.

Throughout Science Week, Dr Cleary is impressing a number of 2FM DJs with a series of fun science experiments.

The science behind the trick ... explained

Oil and water don’t mix. No matter how much you shake them up together, although the oil may break up into smaller drops, it never truly mixes with the water.

Oil is also lighter than water. When you put the glass of water on top of the glass of oil they switch places because the heavier water wants to be at the bottom.

If you put the glass of oil on top of the glass of water, nothing would happen and they would stay exactly where they are!