user: Why do people visit the national parks?

assistant: The national parks are a system of diverse natural and historical locations that are preserved for people to enjoy. Because there is such variety in the national parks there's sure to be something that interests you. Some parks are made up of vast natural landscapes and wildernesses that can be explored. Others are sanctuaries for wildlife where you can see animals in their natural habitat. Still other locations are on islands or have amazing coastline and includes underwater habitats and wildlife. While some national parks are harder to access, most can be enjoyed by the majority of people. If you hate paying high entrance fees to destination locations the national parks are also a great option. You can get an annual pass for less than $100 which allows you to visit all national parks and many national forests and monuments at no additional charge. However, if you want to stay within the national parks make sure to book well in advance as campsites and lodging fills up quickly. For the most popular parks you'll want to book six months in advance when the reservations open up. The national parks can be enjoyed by people of all ages and have stunning vistas you can drive right up to or week long backpacking trips for adventure seekers.

user: Extract the source of silicone carbide in nature.

assistant: <think>
Silicon carbide (SiC), also known as carborundum (/ˌkɑːrbəˈrʌndəm/), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal since 1893 for use as an abrasive. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests. Large single crystals of silicon carbide can be grown by the Lely method and they can be cut into gems known as synthetic moissanite.</think>
Silicon carbide occurs in nature as the extremely rare mineral moissanite

user: Which of the tree species mentioned in the text are used as a source of food for humans?

assistant: <think>
Multipurpose trees or multifunctional trees are trees that are deliberately grown and managed for more than one output. They may supply food in the form of fruit, nuts, or leaves that can be used as a vegetable; while at the same time supplying firewood, adding nitrogen to the soil, or supplying some other combination of multiple outputs. "Multipurpose tree" is a term common to agroforestry, particularly when speaking of tropical agroforestry where the tree owner is a subsistence farmer.

While all trees can be said to serve several purposes, such as providing habitat, shade, or soil improvement; multipurpose trees have a greater impact on a farmer's well-being because they fulfill more than one basic human need. In most cases multipurpose trees have a primary role; such as being part of a living fence, or a windbreak, or used in an ally cropping system. In addition to this they will have one or more secondary roles, most often supplying a family with food or firewood, or both.

When a multipurpose tree is planted, a number of needs and functions can be fulfilled at once. They may be used as a windbreak, while also supplying a staple food for the owner. They may be used as fencepost in a living fence, while also being the main source of firewood for the owner. They may be intercropped into existing fields, to supply nitrogen to the soil, and at the same time serve as a source of both food and firewood.

Common multipurpose trees of the tropics include:

Gliricidia (Gliricidia sepium) – the most common tree used for living fences in Central America, firewood, fodder, fixing nitrogen into the soil.
Moringa (Moringa oleifera) – edible leaves, pods and beans, commonly used for animal forage and shade (it does not fix nitrogen as is commonly believed)
Coconut palm (Cocos nucifera) – used for food, purified water (juice from inside the coconut), roof thatching, firewood, shade.
Neem (Azadirachta indica) – limited use as insect repellent, antibiotic, adding nitrogen to the soil, windbreaks, biomass production for use as mulch, firewood.
Ideally most trees found on tropical farms should be multipurpose, and provide more to the farmer than simply shade and firewood. In most cases they should be nitrogen fixing legumes, or trees that greatly increase the farmer's food security.</think>
Moringa(Moringa oleifera) and Coconut palm (Cocos nucifera) are used as a source of food for humans.