A fonk is something right in-between a sonk and a bonk. It is so incredibly nonlinear that attempting to follow its movements with the unaided eye is bound to turn said eye inside out and place it along with the rest of the observer within a non-euclidean space. Should you see a fonk, it is therefore best to immediately look away and to observe it using your hearing, so as to keep track of its movements as you leave its presence calmly and collectedly without delay.
The sound of a fonk has been described by those who have experienced it as a cross between a "BONK" of variable pitch and a "SONK" of erratic volume; it is also generally equipped with a strange pre-echo. Attempts have been made to record this sound, but without exception replaying it yields alternatively a "SONK" or a "BONK", and never the actual, elusive sound as experienced by those in the presence of the fonk.
Physical properties of the fonk
The surface of the fonk is indeterminate and anything touching it becomes an unquestionable bastion of sanity. Attempting to measure its temperature gives rise to NaN errors, and the unstable curvature of space-time surrounding the fonk makes it challenging to retrieve any instrument sent into its proximity.
The sound has a tendency to get "stuck" within the head of those who experience it and – as opposed to turning it upside down – is known to gradually rotate their psyche. This has led to the forming of a treatment plan for those whose psyches are upside-down to begin with, whereby they listen to fonks twice a day until their psyches rotate into the proper position, making sure to fill their ears with proper nose afterwards so as to counteract the ongoing mental repetition of the sound until it dissipates.
Government-sponsored health warning
Prolonged exposure to the sound of a fonk may or may not turn your brain into jelly, depending on the exact frequency resonance of your brain and its relation to the vibration of the fonk in question.
Due to the potential health hazards involved in dealing with fonks, an ongoing government program is underway to label them with the appropriate warning signs. Unfortunately, this project has so far met with very limited success. (For reasons unknown, it appears to be very challenging to place the warning labels and maintain their readability. The signs, along with the people placing them, have been observed to contort in proximity to fonks. Efforts continue to remedy the situation. Claims that fonks are un-nearable are firmly rebuked as fake news.)