Lime
Sharper and more fragrant than the lemon, and not interchangeable with it in the kitchen. The same job: acid that holds a dish up.
How to use it
Over grilled vegetables, through guacamole, with beans and over fish. In water if you otherwise drink too little.
The best version
Thin skinned and heavy in the hand. A hard, light lime has dried out and gives almost no juice.
How to prepare it
The scent is in the skin and the acid is in the juice, and you want both. Zest first, squeeze second, and add the juice at the last moment: heating it turns it bitter.
What it does
For the lime on its own, little has been measured in people. It is here for the same reason as the lemon: it makes food you would otherwise salt taste good, and it makes water something you reach for more often.
Watch out
Lime juice on your skin in the sun causes burns that look like stains. It is called phytophotodermatitis and it genuinely happens, so wash your hands before you take a lime into the garden.
As with lemon, the acid is bad for your enamel. Rinse with water afterwards and do not brush straight away.