What's the best way to heat a liter of water?

For a project for school we have to heat a liter of water without having it boil. We can use whatever we want but our contraption can't be bigger than 10 inches and has to gain heat/energy from heatlamps. Our best idea is to use a concave mirror to focus the heat in one spot. Obviously we can't use an oven or anything like that. But any suggestions or advice would be greatly appreciated.