Goal and options: wet floor
output answer: could injure a trespasser if it is roped off

Goal and options: To make your free standing bookshelf more stable.
output answer: Attach a premeasured sheet of plywood to the back of the bookshelf using a hammer and nails.

Goal and options: To firmly set pieces of PVC joined together by elbow joints.
output answer:
Use a mallet to hammer the PVC into the joints.