Rub a balloon on a woolen fabric to pick up some electrons, to make the balloon negatively charged, and stick them to a wall, which would be positively charged to make them stick.
Opposites attract and when you stick a negatively charged objects to positively charged objects, they tend to stick together. When you pick up electrons, it increases the number of electrons which will make the object negatively charged.
Note: The first part of the answer is a single sentence. The problem says in a complete sentence, so just in case that you need only one sentence you can take the first part. If you can add in more than a sentence, you can put in more from the second paragraph.
