I loved that little red water rocket and the associated white pump.
Well-made and of extremely hard plastic, I could pump so much pressure into that bottle my 4 year-younger brother would howl on impact.
You had to learn, however, to stand far enough away from your target to allow maximum acceleration while maintaining aim.
There was an art to this.
I also had the red and white variant, and the super long point-y one.

Now you can just make your own, and out of a much softer bottle too! This kit comes with stabilizing fins and bottle end-affixing valves that are compatible with bicycle pumps. While there is one included bottle, you can make your own rockets out of any bottle the valve fits.
This kit, a bike pump, some Duck Tape, and a lot of sugary soda?
My brother's kids will love this.