you'll need:
banana
needle and thread

(1) start at a corner and sew to the next
Starting at one ridge and coming out at the next, push the needle all the way through.

(2) continue around reusing the same holes
Insert your needle into the same hole where the string comes out but angle so the needle exits at the next corner.

(3) finish the loop and pull
After you sew all the way back to the starting point you'll have a loop INSIDE the peel. Grab both ends of the string and pull the loop through the banana core. Repeat this several times for multiple slices.

(4) open and enjoy
It's got to be the slowest way to cut a banana, but if you make up a good story before opening it in front of someone all your effort will be rewarded.