DND_Reborn
The High Aldwin
Sure. It is easy. Rolling a 20 on the d20 does nothing special.Care to expand on this - I'm not sure of what you mean, but it's very intriguing !
(EDIT: THIS just gave me an idea. We allow martial weapons to have advantage on damage, but I like the idea that the natural 20 would give you advantage on damage instead. I'll broach this to our group next session.)
However, rolling maximum damage on the damage die makes the damage die exploding, allowing you to roll an additional die and add it to the damage. If the additional damage die is ALSO maximum, you continue the process.
I used a torch as a club (improvised) once for 21 total damage because I rolled 4 (on d4), 4, 4, 4, 2 + 3 STR mod = 21. It was pretty awesome.

Also, we have special optional features for when you roll critical damage. They require a save by the target with a DC equal to 8 + your attack modifier:
Dazed (for bludgeoning): WIS save or stunned until start of target's next turn.
Hindered (for slashing): CON save or target has half speed until the end of its next turn.
Skewered (for piercing): DEX save or weapon is stuck in target. On your turn you can automatically deal weapon damage (no attack roll required) unless the target uses its action to remove the weapon.
Wounded (for cleaving): CON save or take unmodified weapon damage at the stat of the target's next turn.
All told, this makes using smaller-die weapons a viable alternative to larger weapons because you are more likely to roll maximum (and thus critical) damage.
I should also mention features which grant additional dice on crits work the same (and these can also explode), Champion Fighters (who normally expand the critical range to include 19 and even 18) get to roll one extra die for each expansion on the critical range.
Also, this ISN'T just weapons--this is EVERY SOURCE OF DAMAGE in the game: sneak attacks, smites, traps, falling, spells, you name it! To counter this, dice explode when you roll for healing spells and items.
Lastly, if you go to 0 hit point from critical damage, it is harder for you to recover and you will fall unconscious automatically instead of just being incapacitated IIRC. I'd have to check our current house-rules.