I am a poor, wayfaring stranger. Just traveling through this world of woe. (AKA GBreakdown or Archaick Soul)
今、日本語を勉強しています。 会話レベルには達していません。 まだまだ仮名を暗記しています!
Don't microwave a cup of water by itself for more than a minute or two because of the risk of superheating! This is when a liquid is heated to a temperature higher than its boiling point without actually boiling, and can happen in a microwave because the steam bubbles aren't able to form under a flat undisturbed surface in a smooth container.
Nucleation sites like inserting a wooden spoon or a chopstick into the cup give the bubbles an anchoring point to form, so you can safely boil the water in the microwave. Otherwise, the superheated water in the cup can suddenly flash into steam or erupt boiling water everywhere from just a simple bump like from picking it up. This is what the microwave spoon sticker is trying to convey.