Jeff Foxworthy has a joke that Mom's spit is the same chemical composition as Formula 409. 

Well, in a little bit of child-having today, my 4-year-old child found a fine point sharpie and wrote on our releatively new 22" LCD monitor connected to our computer.  I was worried it wouldn't come out and started google-ing ways to clean it off when my wife spit on her finger and rubbed the perminant marker right off.  While I've been writing this blog, she's been scrubbing with her thumb and some special mom-spit and it's all gone! So, yes, Mr. Foxworthy, My wife's spit must have the same composition as formula 409.  Also, in a weird turn of events, it appears Dad spit is relatively the same.  Though not as potent, my own spit was just as effective in removing the ink… it just took longer.  

It appears that funny jokes must have some truth behind them.  I guess that's what makes them funny.