A triangles puzzle

Taking into accout the decreasing popularity of my puzzles, this is the last one.
What was your mark in geometry in school?

Here u see two triangles.They both consist of 4 figures. The area of figures is the same, and u can check it. But what will happen if we mix them?

How is it possible? Where did the empty square come from?

I really dislike this one its neither optical nor mathematical mystery and its a lie from the very first sentence

fu*k
this one was rly too easy for you
Ann, Newgen (o_O), Baboow, Cinc, memento gave right answers immediately

Anyway there could a topic for this stuff, where ppl would spam random brainteasers

do you propose to write all these puzzles in one topic?
u feel the lack of place in off-topic area?

i noob in babo but i can solve that