Simple miniature homopolar motor using 1,5V clock battery, non-enamelled 0,8 mm copperwire and two 10x3 mm neodymium magnets
Step 1: The Copper Wire
7-8 cm long piece of copper wire is enough. I used a Ø0,8 mm thick, non-enamelled copper wire. If the copper wire is enamelled use a box cutter or other sharp knife to scratch the enamel off from the parts that are in contact with the magnets and the battery.
Use pliers to bend the copper wire. I started with the center tip part that is in contact with the battery. To both sides from the center tip you need to make an 90-degree angle (distance 5-6 mm from the tip). Lock the plier heads and use your fingers for bending. Once you have this M-shape ready, you need to make another 90-degree angles - one in other direction than the other. This distance is determined by the length of the tip. The idea is that the curved part of the wire touches the magnets. For bending the curved part some round surface with a diameter of 13-14 mm is a great aid..
Step 2: Clock Battery and Neodymium Magnets
Then you need one 1,5 V clock battery (Ø12 mm x 5 mm)
a disc neodymium magnet about Ø10 mm, I used two Ø10 mm x 3 mm magnets
Then just pop the bended copper wire on the battery and magnet.
I'd give this one a 2/10 difficulty of realization.