When a lighted matchstick is shown before another light source, the shadow of the matchstick is formed on a screen but not the shadow of the flame. Why is it so?

When did Man learn to make fire himself?

When did writing begin?

When did the accurate mechanical clocks appear?

When did the cultivation of plants begin?

When was iron discovered?

When do otters make use of seaweed?

What                    Why                    When                    Where                   Which                   Who

             How                  Does                  Do                  Is                 Can                 General