Thermochromic ink in action.

From the BBC:

The age-old argument over the best way to cook the perfect boiled egg could be a thing of the past thanks to a new hi-tech ink logo going on shells.

After cooking begins, an invisible, temperature-sensitive thermochromic print appears in black to indicate when an egg is soft, medium or hard-boiled.

Read the full article. Cool!

[tags]eggs, thermochromic, ink, technology[/tags]