Things You'll Need
Rough file or concrete
At some time in life, you may need to make a knife out of a toothbrush. You could be stranded in the wilderness without tools and supplies. You might be confined to a prison cell and need a defensive weapon or means of escape. A toothbrush is already made in the basic shape of a knife, you just have to refine it.
Choose the toothbrush end that will make the best knifepoint. If the handle is thick with a good grip, you may want to remove the bristles and use the bristle end for the knife point, especially if the grip contains rubber. The bristle end may be narrower than the grip end, depending on the make of the toothbrush, which could make it easier to shape.
Find something to rub against the toothbrush to file it down. Concrete works well because it’s rough and can be found almost anywhere.
Create a spearpoint at the knife end by filing the two narrow sides into a point. Consider the hardness of the plastic and don’t create too narrow of a point if the plastic is soft and prone to breaking.
File down the wider sides of the toothbrush. Make the knife blade narrower near the spearpoint, but not so narrow it will break upon impact.
Use something to strengthen the grip of the handle if it isn’t wide and doesn’t already have a rubber grip. You can use duct tape, rubber cement or any substance you think will improve it.
Hide your knife if you need to, but put it in a place where you can reach it easily. To carry it while it’s hidden, slide it up your sleeve and grip the end of it with your palm.