Wudhu

Meaning of reusing water in wudhu

Q: I’ve heard from someone that it is impermissible to reuse water for wudhu, however if someone lets the water from the tap run over his arm such that only one side of the arm is being directly hit with the water and the other side is getting wet by the water flowing over the arm, then will this be a valid wudhu?

Making wudhu after eating food cooked on a fire

Q: There is this book called "The Tablespread Of the Holy Prophet" written by Muhammad Faisal

There is a section on cheese. This hadith is a Sahih Muslim Hadith 687, Narrated by Abu Hurayrah (radhiyallahu anhu).

"Performing ablution in the mosque, he said: I am performing ablution because of having eaten pieces of cheese, for I heard the Messenger of Allah Ta'ala say: Perform ablution (after eating anything) touched by fire."

I would like to know what is meant by "touched by fire" and why should we make wudhu after eating something that is touched by fire?