To make a fruit fly trap--put your bait (piece of fruit, some sugar and water, a pinch of yeast if you have it handy) into a jar. Roll a piece of paper into a cone, with a little opening at the bottom, and tape to hold it together.
Insert the paper into the jar, with the opening above your bait. Flies will come in, but will try to escape from the top of the jar, which is blocked by the paper cone.
When you catch your flies, stick the jar into the freezer overnight to kill the flies.
I just haven't been the same
since that house fell on my sister.