Mountain Bike Tire Question?

Okay, I have a real nice Giant mountain bike that I bought last year, but due to personal reasons I haven't been able to use it for a few months, thus...it's been locked up outside and hasn't been touched. Well, I wanted to finally take it out and I found that the tires were flat, can anyone explain this to me? Why would they go flat and what can I do to get them into working condition....I have a small portable inflating device...will that work or do I need to get new tires? I haven't seriously touched it, nor has anyone else, I don't get why they'd go flat....please help. Thank you.

All tires let air get out over time. So that's normal. Cars tires also does, but they have lot's more air than a bike's, so it's imperceptible.

You will likely only need to reinflate the tires to the correct pressure, and likely that's all you need to do. The only exception is if you stored the bike on the sun for long periods of time, the tubes might have broken or something, but I would just put air in them and then check them again.

The tubes inside your tires leak air slowly over time. The valves aren't perfect and the rubber is permeable as well. Just pump them back up. I generally inflate to the max pressure that can be found on the tire sidewall. If you really need traction to slog through mud, you may want to use a pressure lower than that. Most tires will also have a minimum pressure on the sidewall as well.

What you've seen is perfectly normal. People who ride road bikes where normal tire pressures are 120 psi usually check their tire pressures before every ride.

It's completely normal. Just pump it up.
my Giant XTC 0 is standing in the corner of my room too in flatties because it hasn't been touched for a couple of months and when I'll be able to go for a ride again- I'll just pump the tyres back and let's go!

As the others have said air loss is normal. You should check your pressure before every ride, this can save you a lot of grief on a ride, if your air pressure is low you could pinch flat and if don't have way to repair it on the trail, you might have a long walk ahead.

Just to add a little tech jargon, tubes are usually made from butyl, not rubber. Butyl is air permeable especially in low volume high pressure applications. So, as the bike sits it naturally loses air due to the properties of butyl.

They will go flat over time. If the bike has been left outside for a while you will need to do a critter or junk check on your tire. When left flat for a long time bugs and other objects can get into the spaces between your tire, tube and rim. The best is take the tire off the rim and look for rocks, glass, bugs etc that have made your tire a home. Wipe the inside of the tire with a wet rag, this will pick up any loose dirt, try to turn your tire inside out and run your fingers along the inside, this is one way to find thorns or glass in your tire (it can also hurt too), wipe your rim and look for broken glass, rocks and any tears or damage to your rim strip (a strip of rubber / tape / plastic that covers the spoke nipples and keeps your tube from being cut by the spoke nipples). It is best to use a new tube if possible, if not, pull on the tube, put your hands about 6 inches apart and pull on the tube, this will open up any tears or holes so that they can be seen and fixed, also add some air to the tube and sqeeze the tube together look and listen for air leaking, when found, a suspected leak can be pinpointed with a little spit placed on the hole, bubbles means a leak. If you do not want to go through the trouble of removing your tire / tube, you can still wipe off the out side of the tire and when ready to inflate, stand the wheel / tire up and roll it slowly, pushing down on the top of the tire as you roll the wheel / tire, this will make sure the tire and rim are seated. Then inflate slowly, watch the side of your tire and the rim to make sure that they are seated, a bulging tube or space between the rim and tire means that they are not seated, let the air out and roll the wheel around again to seat the tire to the rim, add air again, if no tube bulges or other issues are found, then inflate to tire spec (written on the side of the tire - min / max), start at the higher pressure end and work your way down in pressure as you ride, tire side wall should flex just a little when you sit on your bike, not enough air and you will get pinch flats and too much air means a rough ride. Also when inflating a tire try to stand to the front or rear of the wheel, standing with one leg on each side works pretty well and try not to bend down on the side of the wheel when adding air, if the tire burst or the tube pops the air and tube parts will fly out of the side of the wheel. Getting a tube in the face is not fun. If un-sure, take it to you local bike shop, have them replace and check your tires and tubes, most will explain what they are doing to help teach you the safe way of tire fixing.