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For using the magnifying glass to set the ants on fire, the ants need to be in the lower right sandy corner, and you just hover the magnifying glass above the ant you wish to set on fire.
As for that making sound/music with ants, this does weaken them, but because you cannot kill the ants no matter how hard and how long you shake the pipette, you can't level them up that way. Do it long enough, and every ant in the pipette will be at exactly 1 HP, where anything that attacks them WILL kill them instantly. However, a plus side, is that EVERY ant that is at 1 HP WILL go back into the hole as quickly as possible.