One of the first skills that any player of NetHack needs to know is how to use the inventory.
Carrying too many items makes you burdened or worse. Also, you are limited to 52 stacks of objects (corresponding to 26 uppercase and 26 lowercase inventory letters), a quantity of gold (which will occupy a special slot labeled '$' if necessary), and, if you pick one up while your inventory is otherwise full, a load stone in a special slot labeled '#,' even if you are strong or the objects all have low weight. If you find a bag, then you can hold more items.
Implicit inventory items
In addition to those items listed with the i command, the hero is assumed to be carrying a wallet or Purse (examinable with the $ command) and a backpack ("A <foo> crawls out of your backpack!") or knapsack ("Your knapsack cannot accommodate any more items.").
The wallet and backpack/knapsack are among the more remarkable objects in NetHack, even though so little is said about them that they go unnoticed by many players. They are utterly impervious to fire, shock, rotting, cursing, or any other kind of degradation. Whatever weight they may have is a matter for speculation only, as they can never be dropped. More worrisome, these items cannot be greased to prevent water damage to their contents, as can any other sack.
The carrying capacity of all containers in NetHack is a subject to inspire much wonder, but the knapsack is perhaps most striking of all containers in this regard: it is capable of holding 52 distinct stacks of items, without regard to how many items are in each stack. If a player were able to contrive to have 52 distinct versions of, say, scrolls, that would be the complete carrying capacity of the knapsack. But, strength allowing, a player could carry one stack containing 52 copies of a single scroll (of known beatitude), and still have room in the knapsack for 51 other stacks of items. A most remarkable container!
It is a popular myth that NetHack characters do not wear pants. However, #sitting on a cockatrice corpse does not turn you to stone, so clearly the hero is wearing some form of pants, trousers or boxer shorts. Veteran roleplayers of TSR's Dungeons and Dragons are familiar with indestructible underwear that is impossible to remove. This tidbit does make them feel right at home in the dungeons of NetHack... until their first encounter with a Foocubus, of course.
Source of fire
Nethack characters can easily light lamps and candles, so it is implied they have some way to make fire. Torches shouldn't last a whole game, unless you carry a large stack or make new ones out of the fat and clothes of slain monsters. Tourists may have a gas lighter. Archeologists are probably heavy smokers (like all fedora-wearing Americans in the 30's) and have a lit cigarette in addition to a petrol lighter or matches. Medieval roles like Knight or Samurai may have matches or some other chemical way to make fire. Cavemen may have some extra flintstones and some dried sponge or mushroom, like the famous caveman Ötzi. Barbarians, who have already invented metal, may have the same kit with an additional fire steel, which makes usage of flintstones much faster. Spellcasters may know some extra fire spell. All in all it's a wonder how characters can light the Candelabrum of Invocation in one move.
In Slash'EM Extended, the player's knapsack can have more than 52 items. Extra inventory items get a # symbol, which can make it difficult to select a specific item to be used because every item after the 52nd one is a #, but it makes dropping a bunch of items on an altar or using a blessed scroll of identify much more useful.