I used to work in a guitar store and around this time every year customers would start coming in to the repair shop with instruments that had been damaged by being left in car trunks on a hot day. Guitars, particularly acoustics, hate heat and a car trunk, or the back of a hatchback, can get up to 160 degrees or hotter very quickly. The excessive heat causes two main problems. The first is that glue breaks down which causes bridges to pull free and braces to pop loose. The second problem is that the wood dries out. The most noticeable sign of this is that the fretboard shrinks and the fret ends start to poke out. It’s also common to see the top seam separate and leave a long split down the center of your guitar’s soundboard. Here are some scary photos at Frets.com of a Martin D-35 that got cooked. And if that wasn’t scary enough, here are some more photos of heat damaged guitars. So how do you protect your instruments from heat damage? Just follow the Dog Rule. Don’t leave your guitar anywhere you wouldn’t leave your dog.  You wouldn’t leave your dog in car trunk so you shouldn’t leave your guitar there.