Hardened paint. Certain types of pigments (especially cobalts) tend to harden in the tube.
Paint hardens because (a) the cap was not screwed on tightly, (b) the paint was stored near excessive heat — over a radiator or in direct sunlight, (c) the pigment was insufficiently "aged" in the vehicle when the paint was manufactured, or (d) the paint is several years old, including both the amount of time you owned it and the time it hung by its neck in a retailer display rack.
If the tube of paint is new or nearly so, request a refund or exchange from the retailer. If you want to salvage a hardened tube, you can do so in two ways.
The first remedy is to force a clear plastic cocktail straw (the narrow kind) into the paint through the mouth of the tube. Push the straw straight into the tube as far down as it will go. Pull the straw out. It should pull with it a plug of hardened paint. If the paint is too hard for a straw to penetrate, you can use a large nail instead.
Fill the hole you've just made with water, and screw on the cap. Knead the tube to mix the water inside with paint, but do not use too much pressure. Set the tube aside for a few days, and repeat if necessary until the paint is sufficiently softened.
The second remedy is to cut the tube open and extract the paint. With a packing knife or sturdy scissors, amputate the empty tube at the crimp, and open the end. Do this carefully, as some parts of the paint may still be liquid. You now can scoop out the paint with a small palette knife (cut down each side of the tube to make this easier).
The hardened paint can be used in several ways. If the paint is still semimoist, the most convenient recourse is to pack the doughy paint into empty plastic dry pans, available from most direct order art retailers (Daniel Smith, Jerry's Artarama, or Cheap Joe's). Let the pans set for a day to dry, then use them in the normal way.
If the paint is so hard it crumbles or breaks when you try to cut it, you can save the dried paint in a small jar, or wrapped in aluminum foil, until you need it for a painting. Dissolve the quantity of paint you need in water. (Usually the paint has to soak for at least day to soften thoroughly, and you may have to add gum arabic or glycerin to adjust the texture.) My preference is just to throw it away.
If the problem recurs, try buying the paint in smaller tube sizes — and use it as soon as possible. Better yet, switch paint brands or art retailer. Well formulated and manufactured paint, displayed and sold by a well managed retailer, stored properly and used within a few years by the artist, will simply not harden in the tube. Period.