Dr Jennifer Cleary shows RTÉ 2fm's Chris Greene and Ciara King that it's possible to push a sharpened pencil through a sandwich bag full of water without it bursting all over them.
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
The sandwich bag is made from long chains of molecules called polymers.
In this case a polymer called low-density polyethylene (LDPE). The polymers are flexible, so the tip of the pencil pushes them apart rather than breaking them.
Because they are flexible, the polymer then forms a temporary seal around the pencil.
If you remove the pencil, the hole stays in the plastic bag because the polymers were stretched apart. qed.