Numbers and digital levels: the number of levels between two adjacent counting units is 10, and each digital level includes 4 numbers. Multi-digit rewriting: first divide it into several levels, then omit the four zeros after the ten thousand digits and replace it with a "ten thousand" word. Multi-digit ellipsis: the mantissa after omitting 10,000 digits depends on the number on the thousand digits and is rounded to the number on the thousand digits. Number generation: the smallest natural number is 0, there is no largest natural number, and the number of natural numbers is infinite. Straight lines and rays: straight lines have no endpoints and can extend to both ends indefinitely, while rays have only one endpoint and can extend to one end indefinitely. Measurement of angle: the size of the angle has nothing to do with the length drawn on both sides of the angle, but with the divergence on both sides. Law of product change: if one factor remains unchanged, another factor will be enlarged or reduced several times, and the product will be enlarged or reduced several times accordingly. Speed, time and distance: speed × time = distance, distance/speed = time, distance/time = speed. Vertical parallelism: Two straight lines that do not intersect the same plane are called parallel lines. Parallelogram and trapezoid: two groups of opposite sides of parallelogram are parallel and unstable. Only one set of opposite sides of a trapezoid is parallel. Quotient is digit: When two digits are divided by three digits, the divisor is compared with the first two digits of the dividend. If the first two digits are big enough, the quotient is two digits; If the first two digits are small, the quotient is one digit. The changing law of quotient: the divisor is constant, the dividend is expanded or reduced several times, and the quotient is also expanded or reduced several times. Reasonable arrangement: total pancake time formula = single-sided time × number of pancakes. You can bake two at a time, and each side has the same time.