There are many ways to gain lives, some of the quicker of which are listed on the GameSpot hints page for this game. In Super Mario 64, you can gain 99 lives easily by talking to Yoshi on the roof of the castle. You need to have all 120 stars to do this.
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in my guide it said if you get 99 lives, Mario takes his hat off. lose your 99 lives and he puts it back on.
You have to get 99 lives
This is what is sometimes called an 'easter egg' which does not affect the game in any way but it's just interesting. Mario does away with his cap (not the propeller cap; he needs that!) when he gets 99 lives in New Super Mario Bros. Wii. 99 is the maximum number of lives you can get.
You have to go to this place in world 3 and keep on jumping at bullet bills until you get 99 lives