Something is thick if it is a larger than normal distance from one side to another. Things like slices of bread are often described as thick. The picture is of some toast with a thick layer of chocolate flavored spread on it.
The opposite of thick is thin or sparse.
Something is thin if there is a very short distance from one side of it to the other. For example, thin ice is ice that would not be strong enough to stand on. The bird in the picture has very thin legs. Another word for thin is skinny. The opposite of thin is fat or thick.
You think when you make words and pictures happen inside your head. You might think about nothing in particular, or you might think about how to solve a problem.
Third comes after second, being the number three time something happens. You might come third place in a race, be third in line, or be like the little girl in the picture, having a third birthday party.
See also thirds.
You are thirsty when you need to have a drink. When you are thirsty your mouth gets dry and you may also get cracked lips and a headache if you go too long without drinking.
Thistles are any plants, usually weeds, that have thorny leaves. Thistles often have thorns all over them, on the stems and flowers too.
The thistle in the picture is the national flower of Scotland, scientific name onopordum acanthium.