Why did the chain on my kid's bike slack after I changed his tire? I changed the rear wheel on my child's bicycle today.  Once I was done and put it back together, the chain is inexplicably longer; there is a lot of slack.

How did this happen and how do I resolve it?



Edit: looking at the photo, I think I may have to move the rear axle backwards.

Edit x2: Photo of successful results, following my suspicions and Paparazzo's answer.
Move the wheel back in the dropout.  Equal on both sides.  Go for 1/2 to 1 inch of play in the chain.

And that valve stem might be leaning a bit.  Probably not enough to matter but ideally it is straight.