In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: Solar water heaters can use energy from the sun to heat water. They are used around the world. The solar water heater is based on a design developed some years ago in Afghanistan. Since then, it has been built and used in many countries. It can heat seventy liters of water to 60degC. It can do this between sunrise and noon on a clear day with an average temperature of 32degC. There are two parts of the solar water heater. One part, the collector, is made of a metal plate painted black. This metal surface, placed in contact with the water, will heat the water. Black-painted surfaces that receive the sun's heat become hotter than those of any other color1. Metal plates that have raised parts on the surface work better than those without them. Once the water is heated, it is kept hot with special material, which allows the water to stay warm for a long time. The second part of the solar water heater holds the water for the system. This water tank can hold about one hundred liters of water. Two rubber pipes are connected to the water tank. One pipe lets water go into the system. The other lets water go out. When the water heater is working correctly, water will flow from the tank to the collector and back again. You can use the hot water at the top of the tank for washing and cleaning. This solar water heater is easy to build and operate. It will last about two years before the rubber pipes need to be replaced.
Answer: can heat water even on cold and rainy days
We can learn that solar water heaters _ .