laser
Explanation
The Joke
Aliens arrive in a spaceship and announce to Earth: "Prepare to be destroyed by the ultra-laser, Earthlings!" The humans look up and one says "I don't get it" while another observes "It's not even powerful enough to reach us." The alien weapon fires, but the laser beam falls short of Earth -- until a cat appears on the ground, spots the laser dot, and chases it. The final panel has someone screaming "NOOOOOOO!" as the cat presumably causes some kind of catastrophe by interacting with the laser.
The joke subverts the alien invasion trope by making the aliens' weapon pathetically weak -- it cannot even reach the planet's surface. But the real threat is not the laser itself; it is the cat's instinctive, uncontrollable compulsion to chase any laser dot it sees. The "ultra-laser" is dangerous not because of its destructive power but because it exploits the one weakness no Earth defense system could account for: a cat's prey drive.
The Humor
The comedy works through misdirection and anticlimax followed by a second twist. The initial setup (alien invasion with a super-weapon) is undercut when the weapon turns out to be feeble. The audience relaxes. Then the cat enters the scene, and anyone who has ever owned a cat immediately understands the danger -- a cat chasing a laser pointer will knock over, destroy, or crash into anything in its path with zero regard for consequences. The "NOOOOOOO!" sells the idea that this trivial-seeming interaction between a weak laser and an ordinary house cat somehow constitutes a genuine existential threat. It is a joke that rewards cat owners with instant recognition humor.