Method 1: Relative Shadows.
On a sunny day you can determine the height of a tree by comparing the length of the shadow of a friend and the shadow of the tree. First measure the length of your friend's shadow and then measure the length of the tree's shadow. The trick is to figure out how many of your friend's shadows it would take to fill the tree's shadow. To do this simply divide the length of the tree's shadow by the length of your friends's shadow. Now all that remains it to multiply your friend's actual height by how many friend shadows you needed to fill the tree's shadow. That will be the height of the tree! Easy peasy.
Method 2: Proportional Height.
This method is very similar to the first method except now the trick is to figure out how many copies of you friend, standing on each other's head, would it take to fill the height of the tree. First, have a friend stand with their back to the tree. Then you hold a ruler at arms length from your eye and walk backwards until the entire tree fits in 10" of the ruler. Measure where the top of your friend’s head (standing by the tree) is on the ruler. Your friend will only appear to be a few inches tall. Divide the 10" of the ruler by the apparent height of your friend. Now measure the actual height of your friend by the tree. Multiply this number by the previous measurement. That will be the height of the tree! Hint: it is easier to make all measurements in inches, then divide by 12 to determine feet. A little harder, but more accurate than the previous method.
Method 3: “Indian” Method
While bending over and looking between your legs, walk away from the tree until the top comes into view. Stop! At this point the height of the tree will be equal to your distance from the tree. Not so accurate, but fun!
Method 4: Tree-Felling Method.
Hold a stick upright in your hand. Walk back until the top of the stick is at the top of the tree and the bottom of the tree is at your thumb. Shout “TIMBER” then twist the stick sideways until it is flat. Note the spot on the ground at the tip of the stick (perpendicular to the trunk of the tree) and have a friend stand on that spot. The height of the tree will equal the distance from your friend to the trunk of the tree.