'Potassium reacts very violently with water producing hydrogen gas which then usually catches fire. Potassium is usually kept under a mineral oil such as kerosene to stop the metal reacting with water vapour present in the air. Unlike lithium and sodium, however, potassium should not be stored under oil indefinitely. If stored longer than 6 months to a year, dangerous shock-sensitive peroxides can form on the metal and under the lid of the container, which can detonate upon opening. It is recommended that potassium, rubidium, or caesium not be stored for longer than three months unless stored in an inert (oxygen free) atmosphere, or under vacuum.

As potassium reacts with water to produce highly flammable hydrogen gas, a potassium fire is only exacerbated by the addition of water, and only a few dry chemicals are effective for putting out such a fire (see the precaution section in sodium).

Potassium also produces potassium hydroxide (KOH) in the reaction with water. Potassium hydroxide is a strong alkali and so is a caustic hazard, causing burns.

Due to the highly reactive nature of potassium, it should be handled with great care, with full skin and eye protection being used and preferably a explosive resistant barrier between the user and the source of the potassium.'

sigh

now i know why