48.11 grams/mole

die

methanethiol is methane gas with one hydrogen replaced by a thiol (sulfur compound)

thiol is pronounced thigh-all or thigh-ole, depending on your accent.

Because they smell bad so you will know if you have a leak.

Asparagus affects the smell of urine, and to a lesser extent, semen. It gives them both a sulphurous smell. Asparagus has sulphur containing products that break down and cause the distinctive smell. To name a few of these chemically ; S-methylthioacrylate and a methanethiol product; S-methyl-3-(methylthio)thiopropionate. Methanethiol is what causes the foul smell in rotton eggs.