Infinite light
In one of those places where you use the sunshine block, hit it, make sure the babies can't see it on their screen, and when there's a little light keep mashing start to open Stuffwell, close him, open him again, and close him again as fast as possible. Then that amount of light is left until you leave, or you get hit by spikes.
More tricks, tips and cheats for this game are right here - Mario & Luigi: Partners in Time cheats
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