Passing a variable from one form to another without creating a new instance of the first form

Question

Let's say I have form1 and form2. Form1 and Form2 are open. You click a button on Form2 and it adds 1 to a variable called myNumber on Form1.

Now the problem is I can't use the new keyword and create a new instance of Form1, because Form1 declares myNumber setting it to 0 on creation of the form. If myNumber gets set to 0 again then all the 1's that were added to that variable were added for nothing....

How do I pass a variable from Form2 to Form1 without creating a new instance of Form1?