Sure, the collar of that undershirt looks fine, but the armpits are caked and yellow like you're developing uranium for a nascent nuclear weapons program. Gross. Apparently the aluminum salts in most antiperspirants mixes with sweat to create a mostly-waterproof stain. The salicylic acid in aspirin makes a nice little anti-yellowcake mixture that can eradicate those sweat stains.
Crush up enough pills and mix with water to create enough paste to cover the sweat stain of your choice. Let it sit for several minutes then rinse. Launder as usual. This treatment is good for any protein-based stain, so pull it out for those times when you get some of that nosebleed on your shirt or dribble egg yolk onto your pants due to over-over-easiness.
Note: pound for pound, this is an expensive method of removing sweat stains (compared to using lemon juice, enzymatic meat tenderizer, or white vinegar), but it's a good thing to bear in mind if you're surprised by a serious stain in a random hotel room that stocks aspirin, but not meat tenderizer, in the lobby gift shop. LOL
