Goo

Goo, also known as Toxic Water, Acid, and Hazard Liquid, is an environmental hazard present in both Portal and Portal 2. Any contact with Goo will result in the player's death. It is usually present in order to provide a penalty for failure, and to ensure that players are precise with their movements and positioning of their Portals.

Goo as it appears in Portal.

“

For this next test, the humans originally requested helmets to avoid brain injuries. I ran the numbers. Making the goo deadly was more cost effective. Click to listen — GLaDOS

”