Why is there only one winter and summer a year​

A proof of that, at the earth's poles, we can only have summer and winter because the limitation of variable (1). ... They are caused by the tilt of the Earth towards the sun in summer and away from the sun in winter. The northern and southern hemispheres both have the same seasons, but at opposite times of the year.

this protocol provides a detailed procedure for co-immunoprecipitation (co-ip) to identify protein–protein interactions.

a chair, a bed, and a toothbrush

explanation: