The premise of the water-rocket car is found in similar toys used as simply rockets. These toys function by pumping water into the object and allowing pressure to force the toy to move away at very high speeds. The idea herein is that the same water-propulsion technique could be applied to a toy car. In this way, a child could pump water, which is easily obtainable, into the back of the car until the pressure finally forces the car away from the pumping mechanism, and to take off at very high speeds. In this way, children could both learn about the basics of pressure, as well as simple kinetic movement. Also, with the toy functioning as such, children could race their cars and have other such contests, as well as build small ramps and other obstacles with which the car can interact.
The construction of the car is projected to be inexpensive to moderately expensive depending largely on the amount of detail that goes into the car. If the car is given an interior, adjustable steering, working steering wheel and the like, production costs would be somewhat higher than that if the car was simply a painted plastic husk. But, all that is really necessary to make the car work would be a pair of metal axles and a fair amount of plastic. This plastic could be formed easily as toy cars are readily available, as well as prototypes for water-pumping mechanisms.
The durability of this toy would be relatively high. If the car were to be made out of a hard plastic, the car would be able to sustain high-speed impacts with little structural damage (though the painting on the car would likely be damaged.) This would allow for any number of accidental collisions, or even purposeful collisions. The pumping mechanism (also plastic), would be able to sustain a fair amount of stress, though the inner workings of the mechanism would be more likely to fail than the simple workings of the car part of the toy. As such, the durability of the toy would be rather high, but could still be damaged if it were to be thrown relatively hard at a hard surface.
This toy would be moderately safe, but not as safe as the other prototypes herein. This is due to the nature of the toy. For one, the car is meant to be launched at very high speeds, which could be harmful to a child standing in the way of the car. Also, any use of pressure is also dangerous. If for some reason the toy were to break due to water pressure, it would be quite likely that the plastic would simply explode, which could cause a fair amount of damage. However, the high speed of the car could easily be replicated if it was thrown, and the likelihood of explosion would be low as the toy would preferably be engineered well before placed on the market. As such, this toy would be very safe, but would be suggested for ages 8 and up, as some sense of self-protection would be necessary to show the child the proper way to play with the toy.