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Needs are your Sims basic necessities. If they are really full, they will earn bonus Simoleons/Experience for an action and each Sim will have green sparkles around them. The 6 needs are hunger, hygiene, bladder, energy, social, and fun. They all can be increased by doing an action that is related to the need (i.e. use the toilet to refresh bladder).
Hunger is the need represented by a crossed fork and knife. It goes down over time, and there is no way to lower it prematurely. It can be filled using items related to food (ex. the microwave, the fridge, the holiday meal tables).
Hygiene is the need represented by a bar of soap. It also goes down over time. However, it can be significantly lowered by shaking your device until your Sims vomit, and then cleaning it up. If your Sim swims in a dirty pool, the hygiene need will be lowered rapidly. The Hygiene need can be filled with bathroom items such as sinks, bathtubs, vanities, and showers. If a Sim's hygiene level becomes too low, a brown stink cloud will appear around them. If your Sim does not use the toilet, their Hygiene will also drop. Another thing that lowers the hygiene bar is performing the WooHoo option repetitively.
The only way to fill a Toddler's hygiene bar is in a bathtub and an adult or teenage Sim is required. The only way to fill a Baby's hygiene bar is by changing its diaper or having a baby go in a bathtub with an adult or teenage Sim.
Bladder is the need represented by a toilet. There is no way to prematurely lower the bladder bar. The bar can only be filled by using a toilet. If your Sim does not use the toilet, s/he literally makes a puddle of pee on the floor which must be swabbed up. This action also lowers the hygiene bar.
Babies are incapable of having this bar filled, as changing a diaper only fills the hygiene bar. It is then advisable to let it lower until it lowers the hygiene bar.
This need is represented by three 'Zs. It can be filled by all chairs and all beds. It can also be filled by a coffee maker and tea maker.
Babies cannot use any beds other than their crib.
The Social need is represented by a speech bubble. Interacting with animals/other Sims, Living Teddy Bears, using phones and computers all fill the bar. There is no consequence for having this bar empty other than loss of extra Simoleons.
This need is represented by a dancing stick figure. It can be filled by numerous objects, far too many to list here. However, some notable examples are garden patches, TVs, books, stereos, tai chi, and toys for toddlers and preteens.