This is fire, burning away...

Fire is a special animation only visible on Blob (View Mode). Fire particles are produced by any material containing Fuel + Hot or Jet + Hot.

The density of the fire, it's bouyancy, and how long each particle lasts can be changed in the Parameters.

For some examples of what can be done by changing the Fire settings, see this Reddit post. To see the images click the link at the top of the page.