Orthocentre of a triangle

Interact with the applet for a few minutes. The purple lines are the ALTITUDES of the triangle. The blue point is the ORTHOCENTRE of the triangle.

What is an orthocentre?

An orthocentre of a triangle is ...

Investigate the orthocentre of a triangle

Move the vertices of the triangle around. What happens with the orthocentre?

When is an orthocentre of a triangle placed outside the triangle?