A bite. All it takes is a bite.
There is a region in the Mushroom Kingdom called 'Wurldwunthruwun' or just "World One" that was formerly used as a supply route for the Koopa Kingdom.
Mario, royal knight of Her Majesty Princess Toadstool, patrols these grounds on a regular basis in search of stray Koopas, mischieving Goombas, as well as those spry, devious Parakoopa Troopas. Uneventful as this duty typically is, one of the Mushroom World's oddities that makes this trip exciting is the series of floating blocks that haunt every district, brook, and valley.
Times having changed since the Great War with the evil King Koopa and his crazed Koopa children, these blocks were once the very citizens of the kingdom transformed by a formidable spell into blocks, weeds, and miscellaneous objects strewn throughout the vast regions. Since Mario had released the evil hands behind this magic, few blocks remained in the wilderness that weren't a product of the MBS (Mushroom Block System; patented by Professor E. Gadd). This invention stylized every "Gadd" block connected to the MBS with a glowing '?' sign emblazoned on the front. The three blocks that hovered before Mario today lacked these.
As any good hero does, Mario decided to check this irregularity, lest it be another plot-in-the-making by some truant Koopas who remained clueless about their master's downfall. The only way to examine floating blocks was the good ol'fashioned way - leaping and bashing one's skull into the bottom of each block with enough power to force their contents out through the top, if not outright shatter the block. Long ago, the Koopa Army made hiding places out of these floating anomalies by hiding coins and provisions inside them until it was safe to return to the Dark Land. Mario popped each block open to find that only dust and insects remained.
Footsteps triggered the Mushroom Hero's finely-honed instincts, spying a pair of Goombas, eyes full of malice as they often were, stalking him from the path he'd previously taken.
Mario disposed of them classically, crushing the two round-bodied agents of Bowser beneath his boot. The now-squished Goombas were never a challenge to Mario, something the Koopa Kingdom learned as their assassination attempts became increasingly pathetic, sending wave after wave of the puny two-directional troops to try and impede him.
Mario started to walk away.
Scraping against the tiled earth, scuttling on the stumps of their legs came more Goombas, or so he thought.
When he turned around, the wave of reinforcements wasn't there.
Just the two little Goombas he'd squashed... still alive, a bit greener than before. Their crushed remains caught their second wind and restarted their attack. Mario squashed them once again, double-stomping them this time to make sure they stayed down, even though it was unusual for a Goomba to get up after being stomped on once.
So Mario left them there, beaten and crushed by his boot, for nature to do with as she saw fit. He started for home, slurping the unsavory taste in the air. Something bit his ankle. The air tasted like earwax.
Mario cried, hopping on one foot, spinning to find his attacker, the Goomba that had gnawed on the back of his left ankle.
The Goomba that bit him no longer had a face, the damage he'd inflicted on it smashed that into a curled, tumor-like mass like an imploded fortune cookie. It was greener than before, all the brown that once identified it changed to the color of plant vomit. Despite all of this, the Goomba kept attacking him. Desperate, Mario leapt, using one mighty foot, violently surging downward with all of his weight onto the relentless enemy.
Crashing down on it produced a result he had never seen before. When he landed, the Goomba wasn't squashed or flattened. No, the Green Goomba shattered. Shattered into a puddle of crimson, fungal chunks from his killing blow.
As he picked up his foot, the Mushroom Hero witnessed the disgusting remains of the Green Goomba splattered under his boot. Tendons and strands wheezed from his boot to the body.
This wasn't the end, for yet another challenger appeared, the second Goomba he'd "squashed", now bearing the same ugly greenness of its dead comrade. It came after him as well. He repeated the killing blow on this Goomba as well, stomping whatever supposed life may have remained in it, pushing the creature's insides out through the Goomba's fleshy surface and coating his shoes an even darker shade of red.
While this gory victory belonged to the Mushroom Hero, an eerie realization banged against his mind. Something was terribly wrong.
Goombas were by definition a mushroom-shaped, living fungus... they didn't have blood.
Confused by what had transpired, Mario clutched his wounded ankle, which was bleeding thanks to the Goomba biting right through his boot. The pain was scalding, nerve-blistering, and it found nothing but delight in torturing poor Mario who ventured all the way back to the Mushroom Castle where a doctor could treat his injury.