its physical science an I have more queations but most of them come with a picture thT you have to answer questions to
The basic answer is this: Electromagnetic Interference (EMI).
However, I doubt that will be sufficient an answer on your homework. So, check out this article: http://en.wikipedia.org/wiki/Electromagnetic_interference_at_2.4_GHz.
Most of today's baby monitors operate at 2.4 GHz. This particular frequency is prone to EMIs from multiple types of devices. Read the article for the meat of why and how it happens.
I hope this helps!