Guess the Gas Evaporated

I normally cut my grass on my off days from my full time job which are Tuesday and Wednesday.  Last week, I went to the store and purchased about 2 gallons of gas for the mower.  My usual routine is to top off before I start, cut about half the yard, top off again and finish the other half.  For those wondering, I have a Poulan push mower.  I added that piece of information for this post to make a little more sense.  Fast forward to this week.  I went outside yesterday to cut the grass.  I went to my gas can to top off what was in the mower.  The can was empty.  I scratched my head for a minute trying to remember if I purchased gas last week because if so, then there should have been some remaining.  I remember getting gas because the store was in the process of lowering the price when I was there.  I didn't kill any ant beds with gas because I have some actual fire ant powder.  My weed eater is now electric and I did not use the blower.  Therefore, I should have had some gas remaining.  I called the nephew to ask if he knew what happened to the gas in the can because I remember him asking for gas money last week.  He told me he didn't know anything about it.  So I guess the gas just evaporated from the can and I am just plain stupid.