To illustrate that medians of a triangle concur at a point called centroid, which always lies
inside the triangle.

Solution;
we know :  Median of a triangle is a line that join a vertex to mid point of opposite side . 

And
Where all three median meet that point is  called cetroid  . 

Now to illustrate the centroid always lies inside the triangle by paper folding , we use these steps  ,

Step 1 -  Cut any shape triangle  . 

Step 2 - Now fold that triangle as we get a line between a vertex and mid point of opposite side  .

Step 3 -  follow step 2 to get three median , now we observe that the three median meet inside the triangle  .