Who keeps taking my pens???

OK how annoying can it get?  Seriously?   I turn around someone is helping themselves and taking my pen.  I just bought a brand new box of pens not even a month ago, I kid you not and they are all gone but this last one I have out now.  Then all of a sudden the entire family has amnesia or think I am crazy.  I have to use a fat pen, not those skinny ones, with a good grip to them.  I think I know how I will nip this in the bud once and for all, I am going to go look for some exploding ink or invisible ink pens that will teach them!!!