A childhood belief about basements

When i was about 4 my sister and i would play in the basement of our house. We had this kitchen set and there was a blue plastic knife in it. My sister put the knife in this little slot where the walls are supposed to meet, but didn't. Anyways the next day the knife was gone and no one knew where it was. My sister told me that my mom told her the monster in the wall took it in the wall with him, and mom didn't tell me cause she thought i'd be scared. Needless to say was afraid of that wall from then on. my sister also told me i could fly....errrr why did i believe her