I had some heat-shrink tubes to shrink around a connector for the wiring for lights my new bicycle, but no heat gun. Since I almost never need to use a heat gun, and didn't want to spring for the expense, and was too shy to ask my handy neighbor if he might have one that I could borrow, I wondered if it might be possible to use an ordinary magnifying glass to concentrate the sun's energy on the tubing and thereby shrink it.
Not owning a magnifying glass either, I purchased a $4 folding one from the local drugstore, which was about 2 inches in diameter, so not terribly large. I figured, if my idea didn't work I'd still use the magnifying glass when I got older and my eyesight deteriorates.
About 2 pm on a sunny and somewhat cool fall afternoon, I wheeled my bike out into the sunlight and angled the magnifying glass to focus the sun's rays on the heat-shrink tubing. I held the magnifying glass toward the sun a few inches from the tube and moved it around until I could see a bright spot of light focused on the tubing.
To my amazement and glee, it worked like a charm. In fact, I was worried it was working a little too well because I could sometimes see wisps of smoke coming off of the tubing. I rotated the wire around and up and down to get the tubing shrunk all the way around the connector and kept the connector moving so as to not let any part overheat and catch fire.
I'd never played with magnifying glasses as a kid, nor ever used a heat gun to shrink heat-shrink tubes. So it is hard for me to compare the actual experience with that of using a heat gun, but in any case, my scheme worked. The tube shrunk quickly before my eyes, and made a tight seal against the connector. It took a minute at the most to get the whole job done.
It was neat to harness the sun's energy in this way. It definitely underscored for me the power of the sun and the danger of letting kids play with magnifying glasses. I was smug with my own ingenuity at saving money and not having to find space to store yet another large and seldom-used gadget at my house, which would have been the case if I'd gone out and bought a heat gun. I felt like a super environmentalist, not having to use electricity, doing what many folks might not have thought possible, etc.