Why do mk whistles have brass tuning slides?

Some players blow harder than others and this affects the pitch of the notes. Whistles are also affected by the air temperature around them - even after warming them up by playing. Mk whistles have tuning slides so you can fine tune your instrument to your playing style and keep your instrument in tune when you're playing in a snowstorm.

Brass is used for the tuning slide on mk whistles because it's hard and inert. In other words it doesn't corrode giving off particles which then cause it to jam.