First of all, you should gain an understanding of their level of English, many non-native Speakers actually have a native level of English.
If you feel that your speaker struggles to understand you, you should speak clearly, not use slang or short forms (cannot rather than can't, going to rather than gonna) and use more simple words.
Because the molecules that make them up move closer together and become more compact
<span> The lumbee indian tribe used walking canoes and horseback for transportation but many tribes did not have horses till the 1700"s
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They allow archaeologist to picture what life could of been like
