The best inventions are not stimulated by great technical skills or ambition, but by frustration (see this essay entitled "No User Parts Inside" on the power of empowered customers). These eleven vexing challenges have frustrated millions of people- find a solution and the world will truly beat a path to your door. But think twice about the answers! Despite many false starts and pretender technologies, no cost effective, practical or acceptable solutions have entered the marketplace. They are extremely tough nuts to crack.

• Invisible Doors- In most homes and apartments, a surprisingly large fraction of each room is lost to a swinging door. The door blocks at least one wall, takes up at least a 3'x3', and never swings the "right" way. The door's path often completely dictates bathroom layouts, and pulling a door towards you, while walking forward, is an unnatural act. A better door could make dorms and apartments significantly more livable. Pocket doors save space, but require enough space in the wall cavity for storage, and some people find them hard to open. Accordion doors jam and frankly are an eyesore. A better door would operate smoothly, take up little space open, block sound when closed, and be aesthetically pleasing. It might even function like a "Dutch door", to hold in (or out) pets and children. Perhaps a miniblind descending from the top jamb? Three panels sliding down from a pocket in the header?
  (Since posted, designboom held a contest on exactly this challenge).

• High strength concrete- Concrete is extremely weak in tension, and will crack almost immediately. So most long concrete spans are "pretensioned" with steel cables. The beam is cast over tense cables, and when cured, the cables are released, transfering their tension to the concrete. This pretension has to be overcome before the beam cracks.
  Clearly, a complex system limted to simple geometries. What if you could mix small wires into the concrete during molding? After curing, the wires shrink along their lenght, putting the entire concrete mass into tension (or just limit to the surface like tempered glass). A huge materials challenge to balance corrosion, adhesion and tensile properties. Shape memory alloy? Amalgams? (as of Nov 2018 a steel shape-memory alloy has become available)
  
  
• Improved car visor- Ever drive northwest near twilight? The sun is nearly blinding, and constantly shifts from the front windshield to the side window. Every time you flip the sunvisor, however, it bangs into your forehead or forces you to duck. Hardly a positive safety feature. A better solution might be a tiny set of curtains that can slide left or right, perhaps the same electrochromic material used to darken rear view mirrors, etc.
  
  
• Inexpensive, comfortable and durable mattress- Everyone hates mattress stores and mattress salesmen. The prices are stubbornly high for a heavy product which quicklly sags and becomes contaminated with mites. Air mattresses are superior, in principle, but comfortable ones remain expensive. How can you rethink the mattress?
  
  
• Easy stacking dishwasher- The dishwasher is a great invention, and if properly loaded, can save energy, sanitize dishes and clean better than hand washing. But, cups often flip over, filling with water and soaking the dry dishes when the dishwasher is opened. And you have to be a jig saw puzzle expert to fill the washer so each dish can be exposed to the sprayer. How can a better dishwasher be designed? Use compressed air to empty water-ladened cups? Hold the plates in netting?
  
  
• Truck Tropical Storm- Its bad enough to drive on the highway in the middle of a rain storm, but 18 wheelers create their own, violent local weather. Trying to pass a tractor trailer at 60 miles an hour is like driving in a hurricane- their wheels fling water into a high speed cloud whirling around the truck like a tornado. Very dangerous. One solution is self-draining porous asphalt, which dramatically reduces surface water and thus spray. But its expensive and not widely used. How can you solve the problem on the truck? Side fenders to suppress the spray? New aerodynamic trailers? Different tires?
  
  
• Quiet lawn blower- Raking leaves and grass is a task from the past- today everyone owns a lawn blower. But these personal jet engines are too loud to use on a quiet Sunday morning, and are  banned in some communities. They may even cause hearing loss. So, find a better solution. For example, it may turn out pulsed air or a string of vortices (like smoke rings) are just as effective, but a great deal more mannered.
  
  
• Microwave Fridge- Microwave ovens elegantly solved the "heat and eat" problem. But what if you wanted to cool things down just as quickly? A cold soda instead of a warm cup of coffee? Or instant Jell-O instead of a steaming bag of popcorn? Perhaps a time reversed acoustic mirror could remove thermal fluctuations- in any case, the physics may not be simple- but is it possible?
  
  
• Solar Cells- Today, solar cells are still about a factor of four too expensive to replace the electrical grid. Cheap cells are inefficient, and the expensive cells are too pricey and complicated to install and maintain. But as many people have pointed out, if you could simply paint on a solar cell.....Don´t forget, a complete solution is a SYSTEM, bringing  solar electricity to the appliance, converting it to 110V, storing it at night, etc. The right solution may involve fine powders for the cell, rather than sheets of silicon.
  
  
• Thermal battery- A couple of bags of Fritos and a soda will keep us running all day long. Our body´s fireplace burns these organic materials pretty effectively, creating heat, electricity and running a sophisticated chemical factory. So why does my laptop battery give out after two short hours? Wouldn't it be fantastic if you could simply crumble up a pack of Saltines, stuff them into a cup on the laptop, add water, and power up for an hour? Not exactly Mr. Fusion from Back to the Future, but the right catalyst and thermoelectric cell could make burning table scraps an alternative to batteries.
  
  
• Snow Melter- There is an old adage- it´s best to fix problems at the source. I used to work in a building which melted the snow off the front sidewalks by running a network of steam pipes under the concrete to keep it warm. On cold days, all the birds and squirrels in the neighborhood warmed themselves on this toasty brick beach. Nice solution, but only one a former monopolist could afford. Any better suggestions to avoid shoveling? A catalytic surface that melts ice? A gel film containing a salt-brine mixture?
  
  
• Geothermal Storage- It's too hot in the summer, and too cold in the winter. But, animals and some humans know the temperature a few feet underground rarely strays from 55 degrees all year long. That´s why basements are usually so pleasant, and why people stored fruits and vegetables in the "root cellar" before modern refrigeration. In new homes, some builders dig a huge network of trenches and lay plastic pipe under ground. By circulating water through this "geothermal storage battery" and then using this water to supplement the heating and cooling system in the house, savings of over 50% in energy usage are possible. But these systems are hard to add to older homes, and often lose thermal contact with the earth, ruining their efficiency. Imagine you froze a whole pond of ice in the winter, and heated a well insulated pool of water in the summer- and then used the store energy in the opposite seasons for temperature control. Not practical in most homes -so how do you build a better geothermal battery?
  
  
• Variable reflective roof- Aesthetic convention demands most roofs are dark or at least gray. This is fine in the winter, when dark roofs absorb much needed sunlight to warm the house. But in summer, half the air conditioner heat load comes from solar radiation. So, why aren´t  roofs white in the summer and dark in the winter? In deference to aesthetic concerns, the color could return to "normal" when the sun is weak... One potential technique is a plastic film I've invented that bends when heated, and thus exposes a more reflecting surface