Since you all brought up Mayonnaise and head lice, I had to go look it up.
Apparently it's to suffocate them, however without putting literally a pound of it in your hair, not sure how that'd work, I mean it's not like it's an oil that spreads easy.
http://www.kidsheadlice.com/mayonnaise-to-treat-head-lice.html
your clothing would be greased out, everything you bump into would be greasy, just ick, why not butter? or even use cooking oil? Curious since some of you are familiar with it, why the choice of mayonaise? What makes that a choice over many other things, including shortening, which would probably not spoil as quick?
Aaron
Oh dear:...
Recommendations vary on how long to leave the mayonnaise in the hair. Some people leave it in only two hours. Dr. Greene, a noted pediatrician, recommends
at least 8 hours. Presumably, the longer you leave it on, the more effective it will be. If you’re leaving the mayonnaise in for a long time, you can apply it right before bed and leave it in overnight.
Sleeping with a head full of mayonaise? Someone's going to be on the couch that's for sure!