Folded paper boats really will float on water: They're an interesting and amusing project for kids, and perfecting boat-folding skills also develops fine motor skills and spatial perception. After you have made a boat or two, you can dispense with the ruler and pencil, as you'll be able to estimate the positioning of the folds well enough to make a good paper boat.
Fold the paper in half widthways, resulting in an 5-1/2-inch wide, 8-1/2-inch-long folded rectangle.
Fold this rectangle in half lengthwise, resulting in a 5-1/2-inch-high, 4-1/4-inch-wide folded rectangle. Press the fold to a sharp crease with your finger. Open out this fold.
Position the rectangle so that the first fold is at the top. Measure down the centre crease 4 1/4 inches and mark the spot with a pencil. Label this point A.
Flip the folded paper over, keeping the fold at the top. Label the top left corner B, and label the top right corner C. Flip the paper back to the previous position.
Fold over and down the upper left corner so that the marked C meets point A. Press the fold into a crease. Fold over and down the upper right corner so that the marked B meets point A. Press the fold into a crease.
Fold up a 1 1/4-inch cuff from the bottom upper layer of the folded paper and finger press the fold into a crease. Fold the right and left edges of the cuff over the edges to form a triangle. Flip the paper over, and fold up and crease the cuff on this side as you just did for the other.
Push the bottom cuffed corners of the triangular shaped paper towards each other, allowing the front and back centres to fold out and away from each other along the centre crease. Join the corners together, forming a folded square.
Rotate the square so that the cuffed point is at the bottom, the folded point is at the top, and the fold runs from the top to the bottom and down the centre (the rotated square now looks like a squat diamond shape.)
Fold the bottom, cuffed point up to the top, matching the point at the top. Press the fold into a crease with your finger. Flip the folded paper over and repeat this step on the other side.
Insert a thumb between the layers of the bottom of the folded paper. Hold the front centre between the thumb and forefinger of the right hand, and the back centre between the thumb and forefinger of the left hand. Pull the sides apart, allowing the right and left edges to come together, forming a square/squat diamond shape.
Position the squat diamond shape so that the open end is at the bottom. Grasp the top layer of each side of the top point between your thumb and forefinger and pull them away from each other and away from the remaining central folded point of the paper. Allow the bottom points to pull up and towards the top centre point, making a folded boat shape. Finger press all visible folds into creases, then pull the sides of the paper boat slightly out and away from the central 'sail' point in the centre of the boat.
The paper boats will eventually disintegrate when they get too wet, but they'll stay sturdy and afloat long enough for boat races on a moving stream or river, or in a wading pool on a breezy day.