I'm helping my daughter with her art class and find it very easy. My last homebrew turned into vinegar. 
Well, I need you to come finish airbrushing the nose-art on my boat then, because I really suck at it. I have a race, this weekend and really need to finish. Would you like to play with my dinghy?
The beer I can help you with. Acetobacter (a genus of acetic acid bacteria) has combined with oxygen to conspire in converting your ethanol into acetic acid, the main component (~8%) in vinegar, aside from water. Although there are other types of anaerobic bacteria that will do this, this is the most common. It happens after primary fermentation (it first needs the ethanol to convert) when you transfer to a secondary fermentation vessel for dry-hopping, etc. The secondary vessel should be filled completely, or purged of all oxygen. I do this by transferring to a secondary before vigorous fermentation has stopped in the primary and before all the yeast has flocculated, thereby allowing the co2 created by the fermentation to purge the container of any remaining oxygen.
If your beer has soured in the primary (not just vinegary, but mouth-puckering sour), it is more likely that you have a lactobacillus infection (Belgian Lambic beer and Berliner Weiss) in which case, you had better practice a stricter sanitization regimen
