Question: A question about the "Explore" command

Greetings!

Strange things happen with the "Explore" command! Let's say I want to plot the function F(x)= a*x. When I am writing:

>Explore(plot(a*x,x=0..10),parameters=[a=0..2]) ;

everything is OK, but when I use:

>F:=a*x;
>Explore(plot(F,x=0..10),parameters=[a=0..2]) ;

the initial frame is correct but when I try to move the parameter cursor, the plot dissapears. 

Any ideas what is wrong and how I can fix this?

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