The Weirdness Bubbles (also known as Bubbles of Pure Madness) are things of evil strange magic used during Weirdmageddon, that make whoever they touch go crazy and also contain different weird animates.
In "Weirdmageddon Part 1" once Bill Cipher and his allies came to Earth, he brought forth bubbles of madness one of them goes by Sprott which make him go insanely mad. As weirdness continues spreading more bubbles are shown floating all over Gravity Falls.
When Dipper and Wendy and are in a car trying to get away from Gideon and his team, they drive through some of the bubbles and as they enter, their heads become bird heads. Dipper and Wendy then go through a couple more and turn into anime characters, meat and their live-action selves.
While both Gideon and Ghost-Eyes drive into the bubbles, they become 8-bites, women and black and white characters.
The bubbles come in many different colours and each contain unimaginable things from within.